Sleep is important. We spend a third of our lives doing it.
I don’t think you need me to tell you that.
The last few years seem to have been characterised by every online expert offering everyday reminders. My particular favourite quote is from Professor Andrew Huberman.
“If you want to know about someone’s health, ask them how they sleep.”
It doesn’t end there. We can now track our sleep and get daily feedback on how we’re doing.
This level of insight is excellent, and I use it myself. However, how much do we know about the different sleep cycles and why they’re essential?
Getting to know our sleep cycles better is what we will tackle in this article.
Various parts of the brain play a role in helping you get to sleep and stay asleep.
For example, the hypothalamus controls sleep onset, the hippocampus and amygdala are active during dreaming, and the reticular formation regulates the transition between sleep and being awake.[1]
Neurotransmitters such as GABA are chemicals that help to promote sleep. GABA works with the brain to inhibit the wakefulness chemicals that keep you up and going during the day. You might have heard of some of them; for example, serotonin and dopamine are both excitatory neurotransmitters that help you get the best out of your day. They don’t help you so much when it comes to sleep.
When the time comes for sleep, GABA comes in.
It’s so pivotal to sleep that three of the most typically used drugs for sleep are based on amplifying GABA’s effect on sleep promotion.[2]
More recently, using GABA as a supplement has helped my patients and numerous people improve their sleep quality.[3]
Another important chemical involved in sleep called adenosine is lesser known but equally important. Adenosine also inhibits the excitatory neurotransmitters in the brain. Whilst it’s not as well-known, its interaction with sleep and the brain affect people globally every day.
How?
Caffeine blocks adenosine in the brain, which partially induces the stimulant effect we love. Unfortunately, doing so can make it difficult to sleep at night, especially when we have coffee or caffeine late at night.[4]
Adenosine builds up over the day and helps promote sleep and GABA.
It’s suggested that the better the buildup of adenosine over the day, the better chance of good sleep quality.[5]
This buildup of adenosine is an excellent example of how the day affects the night when it comes to sleep. Before sleep has even started, your choices influence the quality of it.
Another example is the time you go to sleep. A systematic review which brought together forty-one studies totalling over ninety thousand participants assessed the importance of sleep timing and variability. Sleep timing refers to when you go to bed, and variability covers something called social jetlag.
Social jetlag is relatively new to the scene, first described in 2006. Social jetlag is the discrepancy between biological time, or the time our body thinks it is, and social time, generated by social obligations such as school or work.[6]
Another term for social jetlag is “circadian misalignment.[7] I think circadian misalignment better describes the theme of this study.
The review found two conclusions. The first was that later sleep times and significant variability in sleep, differing durations each night, for instance, were associated with negative health outcomes.
The second conclusion of the review also found, in contrast, that regular sleep timing, or going to bed at the same time every night, was associated with better health outcomes.[8]
All this before you go to sleep!
I wanted this article to focus on the different sleep cycles. However, I feel it’s important to understand just how much has to happen before you even fall asleep to align those sleep cycles.
It’s also important to touch on circadian rhythm because this same rhythm controls the release of different molecules and hormones that affect your sleep cycles!
Sleep cycles occur in five stages.
Once you are asleep, your sleep stages split into two main categories. The first is non-rapid eye movement (NREM) sleep. Believe it or not, there are three stages of NREM sleep, each one a progressively deeper sleep. These stages are called N1, N2, and N3 and occur the most in the first part of the night.[9]
About seventy per cent of our night’s sleep is in the NREM stages, with most of the time spent in N2.[10]
The N1 stage of NREM sleep is the lightest sleep, and we spend the shortest time here during a cycle.
N2 sleep sees your heart rate and body temperature drop and is the stage of sleep where we see memory consolidation.[11]
Specifically, declarative memory consolidation occurs in N2. This type of memory helps you recall facts or events.[12]
Interestingly, the research found that students who study for extended periods had more activity in the N2 stage. This higher activity level then increases interconnections between neurons in the brain as well.[13]
The third and final stage of NREM sleep, N3, is the deepest. It’s the most challenging to wake up during the N3 stage. If you wake up in this stage, there is a good chance that you will experience something called sleep inertia.
Sleep inertia is essentially a period of brain fog after waking up.
By definition, sleep inertia is precisely as it sounds. It’s characterised by reduced vigilance, increased sleepiness and impaired performance.[14] Cognitive studies have shown that people who wake up during this stage of sleep will have moderately impaired mental performance for thirty minutes to an hour.[15]
This negative effect on cognition is an excellent example of how our knowledge of the sleep stages can inform us about our health. I have a large cohort of patients who often confirm waking unrefreshed and feel concerned by this.
What if you were in the wrong stage of sleep at the time?
The results from a comparison between waking during N2 sleep and N3 sleep and performance confirmed this. People who woke from the type of slow wave sleep seen in N3 showed a forty-one per cent reduction in performance compared to before their nap. In contrast, people waking from N2 sleep showed similar performance levels to those already awake.[16]
These changes are another reason why napping beyond thirty minutes is not recommended.
It doesn’t end there, though, as N3 is the stage of sleep where your hormones are most active. For example, growth hormone, which regulates your metabolism, is associated explicitly with N3 sleep.[17]
Why is growth hormone release during sleep significant?
Amongst other things, sleep-related increases in growth hormone relate directly to insulin sensitivity and blood glucose levels. Both affect the quality of your sleep, energy, and brain health the following day.[18]
Stage 3 NREM sleep also plays a role in regulating thyroid stimulating hormone, and some interesting evidence regarding how this combines with insulin in diabetes may go to show more connections between getting adequate sleep.[19]
Aside from these hormonal changes, the body also conducts its repair and regrowth of tissues, builds bone and muscle and strengthens the immune system.[20]
Not only this, but deep sleep also promotes deep cleaning within the brain.
This deep cleaning is all managed by the glymphatic system, the brain’s version of a waste clearance system.[21]
We’ve touched on the different parts of the N3 stage of sleep, but one of the other main functions gaining attention is this deep clean that allows the brain to throw out the trash but also supply the brain with essential resources such as glucose.[22]
Once you have travelled through these three stages, you enter into REM sleep. You spend about a quarter of the night in REM sleep. This stage is often associated with dreaming.[23]
The brain is highly active in the REM stage; by definition, it’s not actually considered a restful stage of sleep. We can see this in the brain via EEG, with the brain waves during REM sleep similar to the ones we have when we are awake.
The REM sleep stage is also the stage when people most often wake up spontaneously.
You’ll go through four to five of these cycles every night.
Each cycle lasts around ninety to one hundred and ten minutes. Interestingly, as the night progresses, your REM sleep increases and your NREM sleep decreases.[24]
Individuals with depression have different levels of REM sleep compared[25] , which speaks to the importance of adequately managing the various stages of sleep.
This importance is one of the key reasons, at least by my interpretation, that the hours before midnight are worth double.
Let’s do the maths as to why.
If we spend most of our time in NREM stage 2, it’s best to consider it a gateway to the all-important N3 stage.
Let’s say that ideally, we want to get five cycles worth of N3 sleep throughout the night, and the minimum length of time for each cycle is ninety minutes. Then the total amount of time is seven and a half hours!
Presuming that we get around eighty per cent efficiency when it comes to how much we sleep in contrast to how long we are in bed, it’s easy to see why eight to nine hours in bed is the gold standard.
My interpretation is that the more cycles one can travel through throughout the night, the better chance there is to strengthen the immune system, reinforce muscle and bone and consolidate learning and memory.
On that basis, we’ve seen in this article that knowing your sleep cycles helps you to become more specific about how long you are sleeping and why it’s so essential for your day!
References
[1] Patel AK, Reddy V, Shumway KR, et al. Physiology, Sleep Stages. [Updated 2022 Sep 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526132/
[2] Gottesmann C. GABA mechanisms and sleep. Neuroscience. 2002;111(2):231-239. doi:10.1016/s0306-4522(02)00034-9
[3] Hepsomali P, Groeger JA, Nishihira J, Scholey A. Effects of Oral Gamma-Aminobutyric Acid (GABA) Administration on Stress and Sleep in Humans: A Systematic Review. Front Neurosci. 2020;14:923. Published 2020 Sep 17. doi:10.3389/fnins.2020.00923
[4] Reichert CF, Deboer T, Landolt HP. Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives. J Sleep Res. 2022;31(4):e13597. doi:10.1111/jsr.13597
[5] Porkka-Heiskanen T. Adenosine in sleep and wakefulness. Ann Med. 1999;31(2):125-129. doi:10.3109/07853899908998788
[6] Caliandro R, Streng AA, van Kerkhof LWM, van der Horst GTJ, Chaves I. Social Jetlag and Related Risks for Human Health: A Timely Review. Nutrients. 2021;13(12):4543. Published 2021 Dec 18. doi:10.3390/nu13124543
[7] Rea MS, Bierman A, Figueiro MG, Bullough JD. A new approach to understanding the impact of circadian disruption on human health. J Circadian Rhythms. 2008;6:7. Published 2008 May 29. doi:10.1186/1740-3391-6-7
[8] Chaput JP, Dutil C, Featherstone R, et al. Sleep timing, sleep consistency, and health in adults: a systematic review. Appl Physiol Nutr Metab. 2020;45(10 (Suppl. 2)):S232-S247. doi:10.1139/apnm-2020-0032
[9] Carley DW, Farabi SS. Physiology of Sleep. Diabetes Spectr. 2016;29(1):5-9. doi:10.2337/diaspect.29.1.5
[10] Malik J, Lo YL, Wu HT. Sleep-wake classification via quantifying heart rate variability by convolutional neural network. Physiol Meas. 2018;39(8):085004. Published 2018 Aug 20. doi:10.1088/1361-6579/aad5a9
[11] Ruch S, Markes O, Duss SB, et al. Sleep stage II contributes to the consolidation of declarative memories. Neuropsychologia. 2012;50(10):2389-2396. doi:10.1016/j.neuropsychologia.2012.06.008
[12] Riedel WJ, Blokland A. Declarative memory. Handb Exp Pharmacol. 2015;228:215-236. doi:10.1007/978-3-319-16522-6_7
[13] Jawabri KH, Raja A. Physiology, Sleep Patterns. [Updated 2022 May 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK551680/
More references
[14] Vallat R, Meunier D, Nicolas A, Ruby P. Hard to wake up? The cerebral correlates of sleep inertia assessed using combined behavioral, EEG and fMRI measures. Neuroimage. 2019;184:266-278. doi:10.1016/j.neuroimage.2018.09.033
[15] Hilditch CJ, McHill AW. Sleep inertia: current insights. Nat Sci Sleep. 2019;11:155-165. Published 2019 Aug 22. doi:10.2147/NSS.S188911
[16] Stampi C. The effects of polyphasic and ultrashort sleep schedules In: Stampi C, editor. Why We Nap: Evolution, Chronobiology and Functions of Polyphasic and Ultrashort Sleep. Boston: Birkhäuser; 1992:137–179.
[17] Gronfier C, Brandenberger G. Ultradian rhythms in pituitary and adrenal hormones: their relations to sleep. Sleep Med Rev. 1998;2(1):17-29. doi:10.1016/s1087-0792(98)90051-x
[18] Van Cauter E, Blackman JD, Roland D, Spire JP, Refetoff S, Polonsky KS. Modulation of glucose regulation and insulin secretion by circadian rhythmicity and sleep. J Clin Invest. 1991;88(3):934-942. doi:10.1172/JCI115396
[19] Goichot B, Brandenberger G, Saini J, Wittersheim G, Follenius M. Nocturnal plasma thyrotropin variations are related to slow-wave sleep. J Sleep Res. 1992;1(3):186-190. doi:10.1111/j.1365-2869.1992.tb00036.x
[20] Patel AK, Reddy V, Shumway KR, et al. Physiology, Sleep Stages. [Updated 2022 Sep 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526132/#
[21] Jessen NA, Munk AS, Lundgaard I, Nedergaard M. The Glymphatic System: A Beginner’s Guide. Neurochem Res. 2015;40(12):2583-2599. doi:10.1007/s11064-015-1581-6
[22] Reddy OC, van der Werf YD. The Sleeping Brain: Harnessing the Power of the Glymphatic System through Lifestyle Choices. Brain Sci. 2020;10(11):868. Published 2020 Nov 17. doi:10.3390/brainsci10110868
[23] Peever J, Fuller PM. Neuroscience: A Distributed Neural Network Controls REM Sleep. Curr Biol. 2016;26(1):R34-R35. doi:10.1016/j.cub.2015.11.011
[24] Patel AK, Reddy V, Shumway KR, et al. Physiology, Sleep Stages. [Updated 2022 Sep 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526132/#
[25] Steiger A, Pawlowski M. Depression and Sleep. Int J Mol Sci. 2019;20(3):607. Published 2019 Jan 31. doi:10.3390/ijms20030607
