How to Improve Sleep and Positively Impact Cognitive Performance

Dr Oliver Finlay

KEY POINTS

·       Mastering sleep is vital for health; American Academy of Sleep Medicine recommends 7+ hours per night for adults, but individual needs vary.

·       Athletes are advised to get 8+ hours, with research showing extended sleep can enhance athletic performance.

·       Sleep affects hormones like leptin and ghrelin, which regulate appetite; inadequate sleep can lead to weight gain and metabolic disruptions.

·       Chronic sleep deficiency increases risk of heart disease, diabetes, and mental health issues like anxiety and depression.

·       Environmental factors, bedtime routines, and diet all impact sleep quality and should be considered for optimal rest and well-being.

Introduction

As concluded in the earlier essay, How Sleep Impacts the Brain, adequate sleep is critical for optimal brain function and cognitive performance. Prioritising sleep is essential for overall health and well-being and should be a priority for everyone.

According to the American Academy of Sleep Medicine and Sleep Research Society, adults should have at least seven hours of sleep per night to avoid the health risks of chronic, inadequate sleep. However, each person is different and some need eight or nine hours to feel rested and alert during the day. Dr Cheri Mah, a sleep specialist at University of California in San Francisco, recommends that athletes have a minimum of eight hours sleep per night and some may need more than ten. Mah consults for teams in the NBA, NFL and NHL and her research with basketball players found that extended sleep can improve free-throw and three-point shooting by 9%.

Sleep plays a crucially important role in both our physical and mental well-being. However, very few of us pay the same attention to improving our quantity and quality of sleep as we pay to enhancing our health and performance through more active pursuits. Get the sleep part wrong, however, and all the good work we commit to exercise, nutrition and mindfulness practices, can be significantly undermined.

Scientists understand that some of our genes act as internal clocks. According to circadian rhythms, which are governed by alternating spells of darkness and light over each 24-hour period, genes are switched on or off, leading to the regulation of hormones such as insulin, cortisol, growth hormone and testosterone. Sustained periods of sleep have a body building effect, with testosterone and growth hormone being released as we sleep. This stimulates the healing and growth of muscle and bone, which help us recover from the exertions of our daily life. When we interfere with these cycles by having too little sleep, our metabolism of glucose declines, which affects our energy levels and our level of the stress hormone, cortisol, increases. Therefore, poor sleep can impair the body’s ability to repair itself and the mind’s ability to regulate mood.

Persistent sleep deficiencies can lead to an increased risk of heart disease, high blood pressure, diabetes, stroke and kidney disease. Sleep deprivation can contribute to anxiety, lack of concentration, poor focus, bad memory recall, slow reactions, reduced productivity and episodes of depression.

The Four Stages of Sleep

Whilst sleeping is often considered a most relaxing pastime, it is actually a time of intense activity in the brain and other parts of the body. Over the course of a night, the body experiences cycles of sleep which consist of 4 phases, ‘Awake’, ‘Light’, ‘Deep’ and ‘REM’, with each phase playing a different role in maintaining mental and physical health through the process of rest and regeneration.

The amount of each phase of sleep can vary from person to person and from night to night, however, in an ideal night’s sleep, the body is able to undergo four to five complete cycles of around 90-110 minutes in duration. Whilst each cycle transitions sequentially through the phases, cycles earlier in the night tend to have more time spent in deep sleep, whilst later cycles have a higher proportion of time spent in REM. Overall, the majority of the night is spent in light sleep and in the final cycle, deep sleep may be skipped altogether (Stutz et al, 2019).

1.     Awake (Stage 0) – this is the natural break in sleep and describes the time spent in bed before and after falling asleep, as well as the brief awakenings that occur during sleep.

2.     Light (Stages 1 & 2) – this phase guides the transition into deeper sleep. During this time, muscles may jerk and then relax, the rate of breathing slows, heart rate decreases, body temperature drops, sleep begins or transitions to the next phase. Waking from this phase is easier.

3.     Deep (Stages 3 & 4) – this phase is when physical restoration occurs, with muscle growth and repair brought about by an increase of blood flow to the muscles, the release of growth hormone, tissue growth and cell repair. Immune function is boosted. Blood pressure drops and the brain flushes waste products and demonstrates long, slow brain waves. During this time waking up is more difficult and results in grogginess or disorientation.

4.     REM (Rapid Eye Movement) (Stage R) – if the previous phase was most focused on physical regeneration, this phase re-energises the mind. During this stage, respiration and heart rate increase, temperature regulation is switched off and vivid dreams may occur and (fortunately, for most people), the body is immobilised to stop you from acting out your dreams. The benefits here are focused on memory, learning and problem solving, as the brain processes information and stores long-term memories.

After a hard workout the body may prioritise deep sleep, whilst if you are recovering from a period of sleep deprivation, the body may prioritise deep sleep for the first few nights to repair the body, before REM becomes the priority to focus on brain regeneration. Caffeine may reduce total sleep time and thus disproportionately cut down on the total time spent in REM, as REM stages are more likely to occur in later sleep cycles.

Sleep and Your Gut

Adequate sleep is very important for a healthy gut microbiome and given the hormonal link between the gut and the brain, this relationship contributes to the impact of sleep on mental health.

The gut microbiome is the community of 400-500 different species of bacterial microorganisms that live in the intestinal tract. Research shows that the microbiome plays a significant role in our overall health through how it affects digestion, improves nutrient absorption and vitamin production, balances hormones (such as cortisol and insulin), impacts the body’s inflammatory response and modulates the immune system (Darian, 2021).

Just like us, the gut microbiome has a circadian rhythm of its own, with gut microbes oscillating in abundance and activity during the day and night. Studies have shown that sleep deprivation is associated with unfavourable changes in gut microbiome composition and can lead to high levels of the biomarker hsCRP, which signals a state of chronic inflammation. In contrast, seven to eight hours sleep a night has been associated with an optimised gut, helping balance and enrich the microorganisms that live within our digestive system and having a positive impact on physical performance, mental health and immunity.

Evaluating Sleep

Poor sleep leads to reduced levels of testosterone, a reduction in insulin sensitivity which can result in elevated levels of blood glucose and increased levels of the stress hormone, cortisol. These changes can be detected by analysing blood samples (Reedy, 2021).

Wearable technology can be used to track aspects of sleep, including sleep duration, sleep quality (by tracking the amount of movement whilst asleep), environmental factors such as the amount of light or temperature in a room, lifestyle factors related to diet or stress levels or the amount of time spent in various phases of sleep (Johns Hopkins Medicine, 2021). As the information collected doesn’t measure sleep directly, it is analysed using algorithms to guess how much sleep has been experienced and can be useful for recognising patterns or trends in your sleep habits.

Clinical sleep studies are the only way of measuring sleep directly and analysing each stage of sleep throughout the course of a night. These studies monitor brain waves and are useful for diagnosing conditions such as sleep apnoea and other disorders.

Sleep Supplementation

Afternoon naps can give temporary improvements in alertness and performance for several hours, but you have to know how to nap properly to get the most benefit. Dr Cheri Mah recommends power naps of 20-30 minutes in duration. Naps of more than 45 minutes allow a deeper sleep, which can result in sleep inertia upon waking, where you feel more sluggish and worn down for a while. Given that caffeine takes about 30 minutes to take effect, having a cup of coffee or green tea immediately before having a nap, will ensure you wake up in a phase of light sleep, feeling refreshed and ready to go again in the right time frame.

Environmental Factors That Affect Sleep

Be intentional about your bedroom environment and make it like a cave…really dark, cool and quiet but also comfortable.

Positive Impact: increased daytime exposure to sunshine; darkness for sleeping (use blackout curtains or eye masks); cool ambient temperatures (~60F/15C) and adequate ventilation increase sleep duration and morning alertness; quiet environments increase sleep duration and quality (the use of ear plugs or white noise can help when environmental noise levels are high); aromatherapy oils such as lavender, geranium, mandarin, bergamot and marjoram promote relaxation and increase the likelihood of falling asleep.

Negative Impact: high humidity (80%+) negatively affects sleep quality compared to lower humidity (50%); prolonged duration of watching television; exposure to bright light or bright screens in the 2 hours before bed (use screen filter settings, apps or yellow tinted lenses to reduce the brightness and intensity of light); dehydration can cause sweats and cramps, whilst hyperhydration can interrupt sleep; inadequate pillows and mattresses – you should change your pillows every 1-2 years and your mattress every 6-8 years.

Foods That Affect Sleep

Positive Impact: tomato products and fatty fish promote increased sleep duration; calcium rich foods minimise non-restorative sleep; tomatoes, sweet potatoes, carrots, brazil nuts, macadamia nuts, coconut oil, dairy and tofu increase the likelihood of falling asleep; magnesium kicks off cascades that bring the body to a restful state that helps you to fall asleep; optimal vitamin D levels (both high and low levels affect sleep quality).

Negative Impact: salt, non-water drinks and foods high in saturated fats and cholesterol decrease sleep quality; alcohol impairs sleep quality; fatty animal products, caffeine and chocolate decrease the ability to fall asleep.

Activities and Routines That Affect Sleep

A regular bedtime, preceded by a 20- to 30-minute wind down routine helps the transition to sleep by quieting the mind. For athletes, this can involve gentle stretching or yoga. Some people like to read a book (chose a real book to avoid screens). Others have a mindfulness practice.

Do: take a warm bath or shower before bed; keep a regular bedtime and a regular pre-bed routine; practice mindfulness or breathing exercises before bed; put your feet up in the evening as the body can start to reabsorb water from your legs before you lie down to sleep and can thus reduce sleep interruptions through needing to empty the bladder overnight; empty your bladder before bed.

Do Not: exercise vigorously in the 2-4 hours before bed; eat big meals in the 2-4 hours before bed; use the bedroom for anything other than sleeping or romance; work on a computer or watch television in the 2 hours before bed; drink large quantities of fluid in the 2 hours before bed; take your phone into the bedroom; drink caffeine in the late afternoon or evening as it has a half-life of about 6 hours and can impact the ability to fall asleep for this amount of time.

References & Evaluation of Scientific Power

Cappuccio, F.P., Taggart, F.M., Kandala, N.-B., Currie, A., Peile, E., Stranges, S. and Miller, M.A. (2008). Meta-analysis of short sleep duration and obesity in children and adults. Sleep, 31(5), pp.619-626.

Overview: The article explores the relationship between inadequate sleep and obesity in both children and adults. The researchers conducted a meta-analysis, which means they analysed and synthesised data from multiple studies to draw overall conclusions. They aimed to investigate whether there is a significant association between shorter sleep duration and increased risk of obesity across different age groups.

Strengths: One strength of the study is its use of a meta-analysis, which combines results from various studies to provide a more comprehensive understanding of the topic. This approach increases the sample size and statistical power, making the findings more reliable. Additionally, the inclusion of both children and adults in the analysis allows for a broader examination of the relationship between sleep duration and obesity.

Limitations: However, there are some limitations to consider. Firstly, while meta-analyses are valuable for synthesising existing research, they are dependent on the quality of the studies included. Variations in study methodologies and participant characteristics could introduce bias or confounding factors. Furthermore, the study may not account for other lifestyle factors that could influence obesity, such as diet and physical activity levels.

Conclusion: The meta-analysis suggests a significant association between short sleep duration and obesity in both children and adults. However, further research is needed to better understand the mechanisms underlying this relationship and to account for potential confounding variables.

Scientific Power: MODERATE - The use of a meta-analysis provides a stronger level of evidence compared to individual studies. However, limitations such as variations in study methodologies and potential confounding factors may impact the strength of the conclusions. Overall, the findings contribute to our understanding of the link between sleep duration and obesity but should be interpreted cautiously.

Comondore, V.R., Wenner, J.B. and Ayas, N.T. (2008). The impact of sleep deprivation in resident physicians on physician and patient safety: is it time for a wake-up call? BCMJ, 50(10), pp.560-564.

Overview: The article delves into the effects of sleep deprivation on resident physicians and its potential impact on both physician and patient safety. Resident physicians are doctors in training who often work long hours, leading to sleep deprivation, which can affect their performance and the safety of their patients.

Strengths: One strength of the study is its focus on a critical issue in healthcare. Sleep deprivation among resident physicians is a widespread concern, and investigating its effects on both physicians and patients is crucial for improving healthcare quality and safety. The article provides insights into the potential consequences of sleep deprivation in this population, raising awareness among healthcare professionals and policymakers.

Limitations: The article may rely on anecdotal evidence or observational studies, which may not provide conclusive evidence of the impact of sleep deprivation on physician and patient safety. Additionally, the study might not account for other factors that could influence safety outcomes, such as workload, stress, or individual differences among resident physicians.

Conclusion: The article highlights the importance of addressing sleep deprivation among resident physicians to ensure both physician well-being and patient safety. While the findings underscore the potential risks associated with sleep deprivation, further research is needed to fully understand its effects and develop effective interventions.

Scientific Power: LOW to MODERATE - While the topic is of significant importance, the evidence provided may rely on observational data or anecdotal reports, which may limit the strength of the conclusions. Nonetheless, the article contributes to raising awareness of the issue and calls for further research and interventions to mitigate the impact of sleep deprivation on healthcare outcomes.

Darian, M. (2021). Optimize your microbiome with the new InsideTracker gut health goal. InsideTracker.com.

Overview: The article discusses the introduction of a new tool called the InsideTracker gut health goal, which aims to help individuals improve their gut microbiome for better overall health. The gut microbiome refers to the community of microorganisms living in the digestive tract, which plays a crucial role in digestion, immunity, and overall well-being.

Strengths: One strength of the article is its focus on a topic of growing interest in health and wellness. Understanding and optimizing the gut microbiome have become increasingly important in recent years, with emerging research highlighting its impact on various aspects of health. The InsideTracker gut health goal offers a practical approach for individuals to assess and improve their gut microbiome through personalized recommendations based on scientific evidence.

Limitations: The article may lack detailed information on the scientific basis of the InsideTracker gut health goal and the specific methodologies used to assess and optimize the gut microbiome. Without transparency regarding the underlying research and evidence supporting the tool, it may be challenging for readers to evaluate its effectiveness and reliability.

Conclusion: The article introduces an intriguing concept for optimising gut health through the InsideTracker gut health goal. While the tool has the potential to provide valuable insights and recommendations for individuals looking to improve their microbiome, further information on its scientific foundation and validation would enhance its credibility and usefulness.

Scientific Power: LOW to MODERATE - While the topic of gut health and the InsideTracker gut health goal are of interest, the article may lack sufficient detail and evidence to fully assess the scientific validity and effectiveness of the tool. Additional research and transparency regarding the methodologies and evidence supporting the gut health goal would strengthen the scientific basis of the article.

Di Milia, L., Vandelanotte, C .and Duncan, M.J. (2013). The association between short sleep and obesity controlling for demographic, lifestyle, work and health related factors. Sleep Medicine, 14(4), pp.319-323.

Overview: The article investigates the link between inadequate sleep duration and obesity while considering various factors such as demographics, lifestyle, work, and health-related variables. This study aims to provide a comprehensive understanding of the relationship between short sleep and obesity, accounting for potential confounding factors.

Strengths: One strength of the study is its thorough consideration of multiple factors that could influence the association between short sleep duration and obesity. By controlling for demographic variables like age and gender, lifestyle factors such as physical activity and dietary habits, as well as work and health-related variables, the researchers can more accurately assess the independent effect of sleep duration on obesity risk. This comprehensive approach enhances the reliability and validity of the study's findings.

Limitations: Despite controlling for various factors, the study may still be subject to residual confounding, where unmeasured or inadequately measured variables could affect the results. Additionally, the study's cross-sectional design limits the ability to establish causality between short sleep and obesity. Longitudinal studies would provide stronger evidence for understanding the temporal relationship between these variables.

Conclusion: The study contributes valuable insights into the association between short sleep duration and obesity while considering a range of potential confounding factors. The findings highlight the importance of adequate sleep for maintaining a healthy weight, even after accounting for various demographic, lifestyle, work, and health-related variables.

Scientific Power: MODERATE to STRONG - The inclusion of multiple factors and the thorough control for potential confounding variables enhance the reliability and validity of the findings. However, the cross-sectional design and potential for residual confounding may slightly lower the overall strength of the evidence. Nonetheless, the study provides valuable contributions to understanding the relationship between sleep duration and obesity.

Johns Hopkins Medicine (2021). Do sleep trackers really work? HopkinsMedicine.com.

Overview: The article explores the effectiveness of sleep tracking devices in accurately monitoring sleep patterns and improving sleep quality. Sleep trackers are wearable devices or smartphone applications designed to monitor various aspects of sleep, such as duration, quality, and stages. This article investigates whether these devices live up to their claims and provide meaningful insights into sleep health.

Strengths: One strength of the article is its critical examination of the efficacy of sleep tracking devices. It discusses the potential benefits of these devices in raising awareness about sleep habits and promoting healthy sleep behaviours. Additionally, the article provides insights into the limitations of sleep trackers, including their accuracy in measuring sleep stages and the potential for user reliance on technology rather than addressing underlying sleep issues.

Limitations: The article may not delve deeply into specific research studies or methodologies evaluating the effectiveness of sleep trackers. Without detailed evidence or references to scientific studies, it may be challenging for readers to fully assess the reliability of the information presented. Additionally, the article may not address individual differences in sleep patterns and preferences, which could impact the usefulness of sleep tracking devices for different individuals.

Conclusion: The article offers a valuable overview of the debate surrounding the effectiveness of sleep trackers. While these devices can provide insights into sleep habits and promote sleep awareness, their accuracy and utility may vary among users. It emphasises the importance of critically evaluating the capabilities and limitations of sleep tracking technology to make informed decisions about its use.

Scientific Power: LOW to MODERATE – The article provides a useful overview of the topic and highlights important considerations regarding sleep trackers, it may lack detailed evidence or references to support its claims. Without specific research studies or methodologies cited, the strength of the information presented may be somewhat limited. Nonetheless, the article contributes to raising awareness and promoting critical thinking about the use of sleep tracking devices.

Lowrie, J. and Brownlow, H. (2020). The impact of sleep deprivation and alcohol on driving: a comparative study. BMC Public Health, 20, pp.980.

Overview: The article explores how sleep deprivation and alcohol consumption affect driving performance. Both sleep deprivation and alcohol impairment are known to increase the risk of accidents while driving, but their comparative impact has not been extensively studied.

Strengths: One strength of the study is its comparative approach, which allows for a direct comparison of the effects of sleep deprivation and alcohol on driving performance. By examining these factors separately and in combination, the researchers can provide insights into the relative risks associated with each condition. This comparative analysis enhances the understanding of the dangers of driving under the influence of alcohol and while sleep deprived.

Limitations: The study may rely on simulated driving tasks rather than real-world driving situations, which could limit the generalisability of the findings. Additionally, individual differences in tolerance to sleep deprivation and alcohol may not be fully accounted for in the study design, potentially influencing the results.

Conclusion: The study contributes valuable insights into the comparative impact of sleep deprivation and alcohol on driving performance. By examining these factors in a controlled setting, the researchers shed light on the risks associated with both conditions and highlight the importance of promoting responsible driving behaviours.

Scientific Power: MODERATE to STRONG - The comparative design allows for a robust examination of the effects of sleep deprivation and alcohol on driving performance. While there may be limitations such as the use of simulated tasks and potential individual differences, the study provides valuable evidence for understanding the risks associated with impaired driving. Overall, the findings contribute to public health efforts aimed at reducing accidents and promoting safe driving practices.

Markwald, R.R., Melanson, E.L., Smith, M.R., Higgins, J., Perreault, L., Eckel, R.H. and Wright Jr, K.P. (2013). Impact of insufficient sleep on total daily energy expenditure, food intake and weight gain. Proc Natl Acad Sci USA, 110(14), pp.5695-5700.

Overview: The article examines how inadequate sleep affects various aspects of metabolism, including total daily energy expenditure (TDEE), food intake, and weight gain. This study explores the potential link between insufficient sleep and weight management, shedding light on the impact of sleep duration on energy balance.

Strengths: One strength of the study is its comprehensive approach to investigating the effects of insufficient sleep on metabolism. The researchers assess not only TDEE but also food intake, providing a more complete understanding of how sleep duration influences energy balance and weight regulation. Additionally, the study incorporates rigorous methodologies, such as controlled laboratory conditions and objective measurements, enhancing the reliability of the findings.

Limitations: The study's findings may be influenced by individual differences in sleep patterns and metabolic responses, as well as potential confounding factors such as diet and physical activity levels. Additionally, the short-term nature of the study may not fully capture the long-term effects of chronic sleep deprivation on weight management and metabolic health.

Conclusion: The study highlights the significant impact of insufficient sleep on metabolism, food intake, and weight gain. By elucidating the mechanisms underlying the relationship between sleep duration and energy balance, the findings underscore the importance of adequate sleep for maintaining a healthy weight and metabolic function.

Scientific Power: MODERATE to STRONG - The comprehensive approach to assessing the effects of insufficient sleep on metabolism, coupled with rigorous methodologies, enhances the reliability and validity of the findings. However, limitations such as individual variability and the short-term nature of the study may slightly attenuate the overall strength of the evidence. Nonetheless, the study contributes valuable insights to our understanding of the role of sleep in weight management and metabolic health.

Patel, S.R. and Hu, F.B. (2008). Short sleep duration and weight gain: a systematic review. Obesity, 16 (3), pp.643-653.

Overview: The article represents a comprehensive analysis of existing research to examine the association between short sleep duration and weight gain. Short sleep duration has been linked to various health issues, and this review aims to provide insights into its potential role in contributing to weight gain.

Strengths: One strength of the study is its systematic approach to reviewing the literature. The authors meticulously search, select, and synthesise relevant studies, ensuring a comprehensive overview of the research on the topic. This systematic methodology enhances the credibility and reliability of the review's findings, providing a solid foundation for drawing conclusions about the relationship between short sleep duration and weight gain.

Limitations: The review may be subject to publication bias, where studies reporting significant associations between short sleep duration and weight gain are more likely to be published, leading to an overestimation of the effect size. Additionally, the review relies on observational studies, which cannot establish causality and may be influenced by confounding variables that are not adequately controlled for.

Conclusion: The systematic review highlights the consistent evidence linking short sleep duration to weight gain. The findings suggest that inadequate sleep may be a contributing factor to the obesity epidemic, underscoring the importance of prioritising sufficient sleep for weight management and overall health.

Scientific Power: MODERATE to STRONG – The systematic methodology ensures a thorough and unbiased examination of the existing literature on the topic, enhancing the reliability of the findings. However, the reliance on observational studies and potential publication bias may slightly lower the overall strength of the evidence. Nonetheless, the review provides valuable insights into the association between short sleep duration and weight gain, contributing to our understanding of the factors influencing obesity.

Reedy, J. (2021). Get a more restful night’s sleep without changing your bedtime. InsideTracker.com.

Overview: The article explores strategies for improving the quality of sleep without necessarily altering bedtime routines. Sleep quality is crucial for overall well-being, and this article aims to provide practical tips for enhancing restfulness during sleep.

Strengths: One strength of the article is its focus on actionable tips for improving sleep quality. By offering strategies that don't require significant changes to bedtime routines, the article provides accessible solutions for individuals seeking better sleep. Additionally, the inclusion of evidence-based recommendations enhances the credibility of the suggestions provided, increasing the likelihood of their effectiveness.

Limitations: The article may not address underlying factors that could be contributing to poor sleep quality, such as stress, environmental factors, or sleep disorders. Without addressing these potential root causes, the effectiveness of the suggested strategies may be limited for some individuals. Additionally, the article's reliance on general recommendations may not account for individual differences in sleep needs and preferences.

Conclusion: The article offers practical advice for improving sleep quality without making significant changes to bedtime routines. While the strategies provided may be helpful for some individuals, addressing underlying factors contributing to poor sleep quality is essential for long-term improvement.

Scientific Power: LOW to MODERATE - While the tips offered may be based on evidence-based practices, the article lacks specific references or empirical data to support the effectiveness of the strategies suggested. Additionally, the general nature of the recommendations and the potential oversight of individual differences in sleep needs and preferences may slightly lower the overall strength of the article. Nonetheless, the practical advice provided contributes to raising awareness and promoting healthy sleep habits.

Stutz, J., Eiholzer, R., and Spengler, C.M. (2019). Effects of evening exercise on sleep in healthy participants: A systematic review and meta-analysis. Sports Med, 49(2), pp.269-287.

Overview: The article investigates the impact of evening exercise on sleep quality in healthy individuals. Exercise is known to influence sleep, but the timing of exercise, particularly in the evening, may have varying effects on sleep outcomes.

Strengths: One strength of the study is its systematic review and meta-analysis approach. By systematically collecting and analysing data from relevant studies, the researchers provide a comprehensive synthesis of the existing literature on the topic. This methodological rigor enhances the credibility and reliability of the findings, allowing for more robust conclusions about the effects of evening exercise on sleep quality.

Limitations: The study may be influenced by heterogeneity among included studies, such as variations in exercise intensity, duration, and timing. Additionally, individual differences in sleep patterns and responses to exercise may not be fully accounted for in the meta-analysis, potentially affecting the generalisability of the findings.

Conclusion: The study provides valuable insights into the effects of evening exercise on sleep quality in healthy individuals. While the findings suggest that evening exercise may have beneficial effects on sleep outcomes, further research is needed to better understand the optimal timing and characteristics of exercise for improving sleep.

Scientific Power: MODERATE to STRONG - The systematic review and meta-analysis approach enhance the reliability and validity of the findings by synthesising data from multiple studies. However, limitations such as heterogeneity among included studies and potential individual differences in responses to exercise may slightly attenuate the overall strength of the evidence. Nonetheless, the study contributes valuable evidence to our understanding of the relationship between evening exercise and sleep quality.

Taheri, S., Ling, L., Austin, D., Young, T. and Mignot, E. (2004). Short sleep duration is association with reduced leptin, elevated ghrelin and increased body mass index. PLoS Med, 1(3), e 62.

Overview: The article investigates the relationship between sleep duration and hormones related to appetite regulation, as well as its impact on body mass index (BMI). Leptin and ghrelin are hormones that play key roles in regulating hunger and satiety, and disruptions in their levels have been linked to obesity.

Strengths: One strength of the study is its focus on exploring the physiological mechanisms underlying the association between sleep duration and weight regulation. By measuring levels of leptin and ghrelin in individuals with varying sleep durations, the researchers provide insight into how inadequate sleep may contribute to changes in appetite regulation and ultimately lead to weight gain. Additionally, the study's inclusion of BMI as an outcome measure strengthens the link between sleep duration, hormone levels, and obesity.

Limitations: The study's cross-sectional design limits the ability to establish causality between short sleep duration, hormone levels, and BMI. It is possible that other factors not accounted for in the study could influence the observed associations. Additionally, the study may not fully capture individual differences in sleep patterns and metabolic responses, which could affect the generalisability of the findings.

Conclusion: The study provides valuable insights into the physiological mechanisms linking short sleep duration to changes in hormone levels and increased BMI. While the findings suggest a potential pathway through which inadequate sleep may contribute to weight gain, further research is needed to elucidate the causal relationship and explore potential interventions.

Scientific Power: MODERATE - While the study provides valuable evidence of the association between short sleep duration, hormone levels, and BMI, its cross-sectional design limits the ability to establish causality. Nonetheless, the findings contribute to our understanding of the complex relationship between sleep and weight regulation, warranting further investigation into potential interventions for improving sleep health and preventing obesity.