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.