Get your Rest: The purposes of sleep.

Humans require sleep. So do all animals, in one form or another. Different animals have different sleep patterns from humans – giraffes sleep five minutes at a time, one half of a dolphin's brain sleeps at a time while the other stays awake – but we all need it. However, sleep is risky because it makes the animal unable to respond quickly to threats. Yet humans and every other animal do it, so it must be important. What purpose does sleep serve, that makes it a necessary part of our daily lives? For many years scientists have understood that sleep helps create and consolidate memories, and recently, they have begun to understand that sleep also helps clean the brain.

Sleep's role in memory

Intuitively, sleep helps memory, and research has long confirmed that. However, describing precisely what is happening – not just that sleep does improve memory, but how and why – is more difficult. 

One of the most important tools used to study sleep is the electroencephalogram (EEG). An EEG uses electrodes attached to the scalp to make broad measures of electrical activity within the brain. Different stages of brain activity, in both sleep and wakefulness, show different wave shapes on the EEG display. These waves are given names based on their frequency: delta (1 to 4 times per second), theta (4 to 8), alpha (8 to 12), beta (12 to 30), and gamma (higher than 30).

These measurements show that there are four stages of sleep. The first and lightest stage is N1, when the beta waves of wakefulness and alpha waves of drowsiness slow down into theta waves. Second is N2, a deeper sleep, which shows bursts of “spindle” activity, about 10 to 15 times per second, that ramp up and then fade away over about a second. Third is N3, also called slow-wave sleep (SWS), the deepest stage, dominated by delta waves. Last is rapid-eye movement (REM) sleep, when vivid dreams occur, associated with theta and beta waves. Throughout the night, sleep cycles through these stages four to five times, in the order N1, N2, N3, N2, REM. 

Even with all the information an EEG provides, it is difficult to translate this broad activity – the aggregated electrical activity of many neurons across large parts of the brain – into specific actions and mechanisms of memory. Research suggests there is a general pattern in which new memories created during the day are located in a part of the brain, deep in the interior, called the hippocampus. During sleep, those new memories are collected, clipped of unimportant parts, consolidated, and eventually sent from the hippocampus to other parts of the brain. 

Different EEG patterns are associated with different stages in this process. For example, spindles during non-REM sleep are thought to occur during the transmission of consolidated memories from the hippocampus to other parts of the brain. Different EEG patterns and their corresponding sleep stages are also associated with different types of memory – slow-wave sleep deals more with declarative memories about specific information, and REM sleep deals more with procedural memories about physical actions and tasks. 

Sleep and cleaning the brain

The human brain weighs only three pounds but uses about a fifth of all the energy the body consumes. All that activity leaves leftovers – spent fuel and parts – waste and debris. Only recently have scientists begun to appreciate the role of sleep in physically cleaning the brain, and removing that waste.

Neurons in the brain are assisted by a wide variety of cells called “glial cells” that support them in their work. Part of that support is forming a tight physical barrier between the brain and the blood vessels within it – the blood-brain barrier. That barrier protects delicate cells in the brain from damage and infection. Neurons and glial cells are therefore in their own “compartment” of space, which is filled with cerebrospinal fluid, CSF. CSF is found throughout the brain, about half a cup of it at any given time, mostly in four connected cavities in the interior, called ventricles.

Researchers discovered that these interconnected ventricles are part of an important circuit called the “glymphatic system.” CSF is not stationary – it flows through the brain. As it does, it carries waste out from around neurons, to the narrow gap between the blood-brain barrier and the blood vessels the barrier surrounds. It cleans the brain out, in a literal sense.

Researchers also discovered that the flow of CSF is not constant. When a person is awake, the flow is very slow. Only when they are asleep – not tired, not resting, but asleep – does the flow reach its highest rate, up to ten times greater than while awake. It is therefore during sleep that the glymphatic system does most of its cleaning.

Recognition of the glymphatic system is quite recent – its presence and activity in humans were only conclusively shown within the past couple of years. Just like sleep and memory, the precise relationship between sleep and the glymphatic system is not fully clear. Studies suggest that disordered sleep degrades the function of the glymphatic system. In turn, accumulations of certain waste products in the brain are associated with various health issues and diseases. For example, beta-amyloid proteins associated with Alzheimer's disease are normally cleared rapidly by CSF flow, and poor flow may be related to their buildup into larger plaques. Vascular dementia is strongly associated with deformation and damage in the space between the blood-brain barrier and the blood vessel through which CSF flows. In general, the glymphatic system has been observed to slow down with age.

Improving sleep hygiene

The precise workings of sleep and the full scope of its functions are not yet clear. However, doctors are certain that good sleep is crucial to good health, and have recommendations for how to get that good sleep – how to improve “sleep hygiene.”

Avoid caffeine, alcohol, and nicotine in the hours before bed. Get exercise, but early in the day, not right before going to sleep. Reduce your exposure to light soon before bed, especially by not looking at screens like on your phone, computer, or TV. Short naps are fine, but longer than an hour or so is not. Cool temperatures around 60ºF are helpful. In general, have a relaxing wind-down routine before bed, and whenever you choose to start it, start at that time consistently. Learning the latest about the study of sleep is great, but remembering these tips is even more important.

Written by Robert Hubbard

Edited by Libby-Ann Sahadeo 

References

1. Rasch B, Born J. About sleep's role in memory. Physiol Rev. 2013 Apr;93(2):681-766. doi: 10.1152/physrev.00032.2012. PMID: 23589831; PMCID: PMC3768102.

2. Arnold, C. Jellyfish caught snoozing give clues to origin of sleep. Nature (2017). https://doi.org/10.1038/nature.2017.22654

3. Mascetti GG. Unihemispheric sleep and asymmetrical sleep: behavioral, neurophysiological, and functional perspectives. Nat Sci Sleep. 2016 Jul 12;8:221-38. doi: 10.2147/NSS.S71970. Erratum in: Nat Sci Sleep. 2016 Dec 20;9:1. doi: 10.2147/NSS.S122160. PMID: 27471418; PMCID: PMC4948738.

4. Burger AL, Fennessy J, Fennessy S, Dierkes PW. Nightly selection of resting sites and group behavior reveal antipredator strategies in giraffe. Ecol Evol. 2020 Feb 14;10(6):2917-2927. doi: 10.1002/ece3.6106. PMID: 32211165; PMCID: PMC7083675.

5. Sakai J. How sleep shapes what we remember-and forget. Proc Natl Acad Sci U S A. 2023 Jan 10;120(2):e2220275120. doi: 10.1073/pnas.2220275120. Epub 2023 Jan 3. PMID: 36595692; PMCID: PMC9926263.

6. Kozlov, M. (2021). Sponge cells hint at origins of nervous system. Nature, 599(7884), 193–193. https://doi.org/10.1038/d41586-021-03015-2 

7. Pappas, S. (2024, February 20). Which creature was the first to take a nap?. Scientific American. https://www.scientificamerican.com/article/which-was-the-first-creature-to-take-a-nap/ 

8. Patel AK, Reddy V, Shumway KR, et al. Physiology, Sleep Stages. . In: StatPearls . Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526132/

9. Sawangjit A, Harkotte M, Oyanedel CN, Niethard N, Born J, Inostroza M. Two distinct ways to form long-term object recognition memory during sleep and wakefulness. Proc Natl Acad Sci U S A. 2022 Aug 23;119(34):e2203165119. doi: 10.1073/pnas.2203165119. Epub 2022 Aug 15. PMID: 35969775; PMCID: PMC9407643.

10. Krajewski, B. (2019, January 14). Human brain facts:. Winston Medical Center. https://www.winstonmedical.org/human-brain-facts/ 

11. Jessen NA, Munk AS, Lundgaard I, Nedergaard M. The Glymphatic System: A Beginner's Guide. Neurochem Res. 2015 Dec;40(12):2583-99. doi: 10.1007/s11064-015-1581-6. Epub 2015 May 7. PMID: 25947369; PMCID: PMC4636982.

12. Chong PLH, Garic D, Shen MD, Lundgaard I, Schwichtenberg AJ. Sleep, cerebrospinal fluid, and the glymphatic system: A systematic review. Sleep Med Rev. 2022 Feb;61:101572. doi: 10.1016/j.smrv.2021.101572. Epub 2021 Nov 18. PMID: 34902819; PMCID: PMC8821419.

13. Xie L, Kang H, Xu Q, Chen MJ, Liao Y, Thiyagarajan M, O'Donnell J, Christensen DJ, Nicholson C, Iliff JJ, Takano T, Deane R, Nedergaard M. Sleep drives metabolite clearance from the adult brain. Science. 2013 Oct 18;342(6156):373-7. doi: 10.1126/science.1241224. PMID: 24136970; PMCID: PMC3880190.

14. Yamamoto EA, Bagley JH, Geltzeiler M, Sanusi OR, Dogan A, Liu JJ, Piantino J. The perivascular space is a conduit for cerebrospinal fluid flow in humans: A proof-of-principle report. Proc Natl Acad Sci U S A. 2024 Oct 15;121(42):e2407246121. doi: 10.1073/pnas.2407246121. Epub 2024 Oct 7. PMID: 39374384; PMCID: PMC11494350.

15. Di H, Guo Y, Daghlas I, Wang L, Liu G, Pan A, Liu L, Shan Z. Evaluation of Sleep Habits and Disturbances Among US Adults, 2017-2020. JAMA Netw Open. 2022 Nov 1;5(11):e2240788. doi: 10.1001/jamanetworkopen.2022.40788. PMID: 36346632; PMCID: PMC9644264.

16. Chunnan L, Shaomei S, Wannian L. The association between sleep and depressive symptoms in US adults: data from the NHANES (2007-2014). Epidemiol Psychiatr Sci. 2022 Sep 8;31:e63. doi: 10.1017/S2045796022000452. PMID: 36073029; PMCID: PMC9483824.

17. Chaput JP, McHill AW, Cox RC, Broussard JL, Dutil C, da Costa BGG, Sampasa-Kanyinga H, Wright KP Jr. The role of insufficient sleep and circadian misalignment in obesity. Nat Rev Endocrinol. 2023 Feb;19(2):82-97. doi: 10.1038/s41574-022-00747-7. Epub 2022 Oct 24. PMID: 36280789; PMCID: PMC9590398.

18. De Pasquale C, El Kazzi M, Sutherland K, Shriane AE, Vincent GE, Cistulli PA, Bin YS. Sleep hygiene - What do we mean? A bibliographic review. Sleep Med Rev. 2024 Jun;75:101930. doi: 10.1016/j.smrv.2024.101930. Epub 2024 Apr 16. PMID: 38761649.