The Physiology of Sleep

Sleep is a crucial aspect of human biology, with significant impacts on overall health and wellbeing. There are two main stages of sleep, NREM (Non-Rapid Eye Movement) and REM (Rapid Eye Movement), each with their own distinct characteristics and benefits.

During NREM sleep, the body secretes hormones such as:

  • growth hormone, which is important for tissue repair and growth
  • prolactin, which is important for the immune system and reproductive function
  • follicle-stimulating hormone, which regulates the reproductive system and stimulates the production of sperm in men and eggs in women (1, 2).

During REM sleep, the body secretes hormones such as:

  • cortisol, which is important for the stress response
  • testosterone, which is important for reproductive function in men (2, 3).

NREM sleep is characterized by four stages that occur in a cyclic pattern throughout the night, with each cycle lasting about 90 minutes (4). Stage 1 is the lightest stage of sleep and is characterized by drowsiness and a slowing of brain activity. Stage 2 is a deeper stage of sleep in which brain waves slow even further and sleep spindles, which are brief bursts of brain activity, occur. Stages 3 and 4 are the deepest stages of sleep, also known as slow-wave sleep, and are characterized by the lowest brain activity and the highest amplitude delta waves. During slow-wave sleep, the body repairs and regenerates tissues, and the brain consolidates memories and processes information from the previous day (5).

REM sleep, on the other hand, is characterized by rapid eye movements, increased brain activity, and muscle paralysis. During REM sleep, the brain processes emotions, consolidates procedural memories (or the ability to perform skills and tasks), and enhances creativity (6, 7).

Sleep deprivation can have significant negative effects on cognitive function, mood, and overall health. Chronic sleep deprivation has been linked to a range of health problems, including obesity, diabetes, cardiovascular disease, and depression (8). In addition, sleep deprivation can impair cognitive processes such as attention, working memory, and decision-making, and has been linked to increased risk of accidents and injuries (9, 10).

Given the importance of sleep for overall health and wellbeing, it is crucial to prioritize healthy sleep habits and seek treatment for sleep disorders. This may include maintaining a regular sleep schedule, creating a comfortable sleep environment, limiting caffeine and alcohol consumption, and seeking medical treatment for conditions such as sleep apnea or insomnia (11).

  1. Vgontzas, A. N., Mastorakos, G., Bixler, E. O., Kales, A., Gold, P. W., & Chrousos, G. P. (1999). Sleep deprivation effects on the activity of the hypothalamic-pituitary-adrenal and growth axes: potential clinical implications. Clinical Endocrinology, 51(2), 205-215.
  2. Kryger, M. H., Roth, T., & Dement, W. C. (2016). Principles and practice of sleep medicine. Elsevier.
  3. Luboshitzky, R., Zabari, Z., Shen-Orr, Z., Herer, P., & Lavie, P. (2001). Disruption of the nocturnal testosterone rhythm by sleep fragmentation in normal men. The Journal of Clinical Endocrinology & Metabolism, 86(3), 1134-1139.
  4. National Sleep Foundation. (2021). Stages of sleep.
  5. Stickgold, R., Walker, M. P., & Sleep, D. (2013). The neuroscience of sleep. Academic Press.
  6. Walker, M. P., & van der Helm, E. (2009). Overnight therapy? The role of sleep in emotional brain processing. Psychological Bulletin, 135(5), 731-748.
  7. Mednick, S. C., Cai, D. J., Shuman, T., Anagnostaras, S., & Wixted, J. T. (2011). An opportunistic theory of cellular and systems consolidation. Trends in Neurosciences, 34(10), 504-514.
  8. Cappuccio, F. P., D’Elia, L., Strazzullo, P., & Miller, M. A. (2010). Sleep duration and all-cause mortality: a systematic review and meta-analysis of prospective studies. Sleep, 33(5), 585-592.
  9. Lim, J., & Dinges, D. F. (2008). Sleep deprivation and vigilant attention. Annals of the New York Academy of Sciences, 1129(1), 305-322.
  10. Killgore, W. D. S. (2010). Effects of sleep deprivation on cognition. Progress in Brain Research, 185, 105-129.
  11. National Institute of Neurological Disorders and Stroke. (2019). Brain basics: Understanding sleep.

Pain and Sleep

A few days ago, Krause et al. from the University of California (Berkeley), published the results of their research on the relationship between pain and sleep. They found that poor sleep quality was linked to increases in pain experience, whereas better sleep quality decreased pain. Krause notes that “the results clearly show that even very subtle changes in nightly sleep – reductions that many of us think little of in terms of consequences – have a clear impact on your next-day pain burden”. In the laboratory, they were able to identify that sleep deprivation amplifies the reactivity in parts of the brain (somatosensory cortex) responsible for the perception of pain. Whilst deactivating the responses from some pain relieving centres (insula and nucleus accumbens).

The irony is that pain often disrupts sleep, and thus the pain experience gets magnified and a vicious cycle is created.

Sleep Deprivation Can Negatively Affect Cholesterol Levels And Inflammation


Vilma Aho et al from the University of Helsinki conducted 2 studies looking into the effects of sleep deprivation. The first study was experimental and consisted of partial sleep restriction to a small group of subjects. The second was an epidemiological study with over 2700 individuals. Blood samples were analysed in both cases.

The analyses revealed decreased circulating High Density Lipoproteins (HDL cholesterol), otherwise known as ‘good cholesterol’, and elevated inflammatory markers. Sleep loss decreased the expression of genes encoding cholesterol transporters and increased expression in pathways involved in inflammatory responses. The findings help to explain why sleep deprivation is a risk factor for cardiometabolic disease.

Bedtime Affects Weight


A group of researchers from Berkeley and Columbia University looked at the relationship between bedtime and weight gain in young people. Their findings have been published in the journal Sleep. They followed over 3000 adolescents between 1994 and 2009. They found that a later bedtime during the workweek was associated with an increase in body mass index (BMI) over time. Even after controlling for sleep duration, screen time and exercise, the relationship was not attenuated. However, fast-food consumption was recognized as a mediator of the relationship. To summarise, if we want to keep off excess pounds we should get to bed earlier and obviously keep away from fast food!

Sleep Quality Linked to Rumination


Jacob Nota and Meredith Coles from the Department of Psychology at Binghamton University in the US have made some interesting discoveries regarding sleep duration and timing. Their research confirmed what others had already noticed, that rumination (repetitive negative thinking) was associated with reduced sleep duration. In addition to this, they found that the timing of sleep was also important. Individuals that reported later sleep and activity times also reported more repetitive negative thinking.

People that ruminate tend to suffer more from anxiety, depression, post-traumatic stress disorder and obsessive-compulsive disorders. For those that sleep too few hours, increasing sleep has already been found to decrease symptoms of psychopathology. Further efforts to get to bed earlier may provide even more benefit.

These recent findings highlight the importance of sleep on health.

Poor Sleep Linked To Pain


A new study by Arthritis Research UK Primary Care Centre at Keele University and published in Arthritis & Rheumatology has identified factors associated with an increased risk of developing widespread pain in adults over 50. Of the factors measured, non-restorative sleep was the strongest independent predictor of new onset widespread pain.

How Does Sleep Loss Lead To Obesity?

Numerous studies have shown a link between sleep loss and obesity. Medical News Today summarises the findings of recent research by Stephanie Greer and her colleagues. They found that brain activity in the frontal lobe was significantly impaired following a night of sleep deprivation. The frontal lobe is responsible for higher mental functions such as “the ability to recognise future consequences resulting from current actions“. It’s thought that the impairment in brain function caused by sleep loss leads to poor food choices…and later to obesity.

This got me thinking…obesity is known to predispose to obstructive sleep apnea (OSA). OSA is a disorder caused by temporary blockages of the airway during sleep. This causes repeated periods when people actually stop breathing…shortly followed by gasps for air. Obviously this affects the quality of sleep and can lead to severe daytime sleepiness.

Could there be a vicious cycle at play here?

Sleep Deprivation

Sleep is vital to our health and well-being. Short term sleep deprivation can lead to decreased coordination and reaction time, decreased attention, irritability, headaches, stomach problems, sore joints, etc. Long term sleep deprivation can lead to obesity, heart problems, diabetes and a shorter life span. The average adult needs 7-8 hours of sleep a night and getting those hours should be one of our priorities. It’s no drama if we get a few shorter nights as long as we make up the sleep debt in the following days.

Pain: how much is in the mind?

A study by Luis Buenaver and colleagues published in the June edition of Pain has shown some fascinating links between pain, thinking and sleep. It’s already known that about 80% of people with chronic pain experience troubled sleep and that poor sleep increases pain sensitivity. It is also known that pain catastrophizing is a contributor to chronic pain. Catastrophizing is a combination of rumination and negative thinking. What Buenaver and his colleagues discovered was a direct link between pain catastrophizing, poor sleep and worsening pain…a vicious cycle…the more often you think negatively about your pain experience…the more your sleep is disrupted…the worse your pain becomes…which in turn adversely affects your thoughts, etc.



Another piece of research published in this month’s Current Biology by Christian Sprenger et al. demonstrates that mental distraction techniques can decrease pain sensations by inhibiting incoming pain signals in the spinal cord. The effect was found to be partly mediated by endorphins.

In essence, people with chronic pain can improve their sleep and comfort by choosing to focus on pleasant activities or goals rather than brood gloomily over their pain.

To change the way we feel we must change the way we think!

Shift Work May Cause Obesity And Diabetes

An article in today’s BBC News Health discusses recent research published in Science Translational Medicine. Dr Orfeu Buxton et al. have shown that “prolonged sleep restriction with concurrent circadian disruption alters metabolism and could increase the risk of obesity and diabetes”. This has implications to people performing shift work.