Medically reviewed by
Dacelin St Martin, MD
Triple board-certified in Sleep Medicine,
Internal Medicine, and Pediatrics.

 
 
Stages of Sleep  |  Mechanisms & Processes  |  Physiology of Sleep  |  Chemical Signals  |  Importance of Sleep

Overview

You have to satisfy the three pillars of health for optimal wellness: diet, exercise, and sleep. Sleep, though mentioned third, is as essential as diet and exercise. 

If you do not get enough sleep, it will negate the benefits of eating healthy foods and exercising regularly. Quality sleep is considered one of the fundamental building blocks of wellness. 

Let’s discuss the concept of sleep in terms of its stages, physiology, and benefits. 

What are the Stages of Sleep?

You can maintain good health by ensuring that you get 7-9 hours of restorative sleep each night. Sleep can be categorized into 4 stages, consisting of REM and non-REM sleep, that you cycle through each night.^[1]

The REM (rapid eye movement) sleep phase consists of one stage of sleep, while the non-REM sleep consists of 3 stages. Let’s explore each of these stages and see what happens physiologically.

Stage N1  

The 1st stage of non-REM sleep occurs when you start feeling drowsy and begin to fall asleep. Sleep is so light that you may not perceive that you’re in a state of sleep.  It generally lasts for a few minutes.^[2] During this stage, your breathing and heartbeat slow down, and muscles slowly begin to relax. 

Stage N2 

Stage N2 is a non-REM phase characterized by light sleep. It lasts for about 25 minutes, which is the largest portion of sleep time for a middle-aged adult.

During this stage, your breathing and heartbeat slow down further, and the eye movements cease to occur. The body temperature drops to create a favorable internal environment suitable for the next deeper phase of sleep. The brain waves spike up and down sharply, producing sleep spindles and k-complexes.

Stage N3 

This is the deepest stage of non-REM sleep during which your body performs several critical health-promoting functions. 

Stage N3 is characterized by slow-wave sleep: breathing rate and heartbeat slow down, eye movements cease, and muscles relax. This sleep stage is critical because your body uses this time to repair tissues and regenerate cells. Arousal from sleep can be more difficult during this stage.^[3]

REM Sleep (Stage R) 

REM stage occurs approximately 90 minutes after you fall asleep. It’s the “dreaming” stage that lasts about 10 minutes during the 1st cycle. REM sleep duration increases with each REM cycle, with the final cycle lasting for about 60 minutes.

During this stage, eye movements become more rapid, and your heart rate and breathing increase. Your muscles become temporarily paralyzed, though they may twitch occasionally. Brain activity increases markedly during this stage.^[4] You cycle through these 4 sleep stages several times throughout the night, approximately every 90 minutes.

Studies suggest that this stage is crucial for memory consolidation and retention, and management of less significant neural connections.^[5] 

Mechanisms & Processes 

Two internal biological mechanisms called homeostasis and circadian rhythm work in sync to regulate your sleep-wake cycle.  

1) The Circadian Rhythm 

The circadian rhythm controls the timing of your sleep-wake cycle. It causes you to feel sleepy at night and wakes you up in the morning at the same time every day, even without an alarm.^[6]

Your body’s biological clock operates on the 24-hour-day-night cycle of the earth. It synchronizes your sleep-wake schedule based on environmental cues, such as light and temperature, to help you sleep at night when it’s dark and wake you up in the morning when it’s bright.^[7]

2) Sleep-Wake Homeostasis 

This mechanism works as an alarm and keeps track of your mind and body’s need for restorative sleep. It reminds your body to sleep after a certain number of hours of being awake. Your sleep drive tends to get stronger every hour when you stay awake and helps you sleep longer after sleep deprivation.

Physiology of Sleep

How many hours of sleep do you need?

A cumulative sound sleep of 7 to 9 hours every night is what is commonly recommended by physicians for adults. Most babies sleep for about 16 to 18 hours a day, as a longer sleep duration is necessary to boost their growth and development. School-age children and teenagers need about 9 to 10 hours of sleep every night. 

Chemical Signals Involved in Sleep     

Your brain is comprised of clusters of sleep-promoting neurons that become active when you get ready for bed. 

The increased secretion of nerve-signaling chemicals known as neurotransmitters in the brain, like GABA, norepinephrine, and orexin can “switch off” or suppress the activities of cells that signal arousal, thus promoting sleep. 

GABA is associated with improved sleep quality, sedation, and muscle relaxation. Norepinephrine and orexin keep some regions of the brain active while you are asleep. Other neurotransmitters like acetylcholine, cortisol, histamine, adrenaline, and serotonin also shape your sleep pattern and regulate the sleep-wake cycle. 

The Importance of Sleep

• Poor sleep is linked to obesity and weight gain, while a healthy sleep-wake pattern can help you maintain a healthy weight^[8] 
• Sleep deprivation may disrupt the secretion of appetite hormones, like leptin and ghrelin, and contribute to increased appetite^[9]
• Good sleep improves your concentration and daytime productivity^[10]
• A healthy sleep pattern is critical for improving your energy levels and physical performance. It significantly improves your accuracy, speed, and reaction times^[11]
• A healthy sleep pattern would reduce your risk of heart disease and stroke^[12]
• Restorative sleep improves glucose metabolism and reduces your risk of type 2 diabetes^[13]
• A healthy sleep-wake cycle can stabilize your emotional and intellectual health, reduce the risk of depression, and promote the brain’s cognitive skills, like memory and attention span^[14]

Conclusion

Your sleep-wake pattern plays a critical role in supporting your health and fitness. If you have issues with getting sound sleep, reach out to your doctor to determine if you have an underlying sleep disorder.

References:

1. Patel AK, Reddy V, Araujo JF. Physiology, Sleep Stages. [Updated 2020 Apr 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526132/
2. Institute of Medicine (US) Committee on Sleep Medicine and Research; Colten HR, Altevogt BM, editors. Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem. Washington (DC): National Academies Press (US);         2006. 2, Sleep Physiology. Available from: https://www.ncbi.nlm.nih.gov/books/NBK19956/
3. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Stages of Sleep. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10996/
4. [Institute of Medicine (US) Committee on Sleep Medicine and Research; Colten HR, Altevogt BM, editors. Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem. Washington (DC): National Academies Press (US);         2006. 2, Sleep Physiology. Available from: https://www.ncbi.nlm.nih.gov/books/NBK19956/
5. Tononi, G., & Cirelli, C. (2013). Perchance to prune. During sleep, the brain weakens the connections among nerve cells, apparently conserving energy and, paradoxically, aiding memory. Scientific American, 309(2), 34–39. https://doi.org/10.1038/scientificamerican0813-34
6. Reddy S, Reddy V, Sharma S. Physiology, Circadian Rhythm. [Updated 2020 Jul 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from:           https://www.ncbi.nlm.nih.gov/books/NBK519507/
7. Huang, W., Ramsey, K. M., Marcheva, B., & Bass, J. (2011). Circadian rhythms, sleep, and metabolism. The Journal of clinical investigation, 121(6), 2133–2141. https://doi.org/10.1172/JCI46043
8. Taheri, S., Lin, L., Austin, D., Young, T., & Mignot, E. (2004). Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index. PLoS medicine, 1(3),  e62.  https://doi.org/10.1371/journal.pmed.0010062
9. Taheri, S., Lin, L., Austin, D., Young, T., & Mignot, E. (2004). Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index. PLoS medicine, 1(3), e62. https://doi.org/10.1371/journal.pmed.0010062
10. Ellenbogen J. M. (2005). Cognitive benefits of sleep and their loss due to sleep deprivation. Neurology, 64(7), E25–E27. https://doi.org/10.1212/01.wnl.0000164850.68115.81
11. Mah, C. D., Mah, K. E., Kezirian, E. J., & Dement, W. C. (2011). The effects of sleep extension on the athletic performance of collegiate basketball players. Sleep, 34(7), 943–950. https://doi.org/10.5665/SLEEP.1132
12. Cappuccio, F. P., Cooper, D., D’Elia, L., Strazzullo, P., & Miller, M. A. (2011). Sleep duration predicts cardiovascular outcomes: a systematic review and meta-analysis of prospective studies. European heart journal, 32(12), 1484–1492. https://doi.org/10.1093/eurheartj/ehr007
13. Van Leeuwen, W. M., Hublin, C., Sallinen, M., Härmä, M., Hirvonen, A., & Porkka-Heiskanen, T. (2010). Prolonged sleep restriction affects glucose metabolism in healthy young men. International journal of endocrinology, 2010, 108641. https://doi.org/10.1155/2010/108641
14. Bernert, R. A., Turvey, C. L., Conwell, Y., & Joiner, T. E., Jr (2014). Association of poor subjective sleep quality with risk for death by suicide during a 10-year period: a longitudinal, population-based study of late life. JAMA psychiatry, 71(10), 1129–1137. https://doi.org/10.1001/jamapsychiatry.2014.1126