Scientists are still not sure about the exact function of sleep, but sensitive measurement tools have provided deep insights into what happens to the brain and body during sleep. Read more about the science of sleep cycles, and what the evidence says about whether we can adapt to shorter sleep.
Sleep stages
Sleep is characterized in two types: non-rapid eye movement sleep (NREM), and rapid eye movement sleep (REM). We alternate between Non-REM and REM sleep approximately once every 90 – 100 minutes. Non-REM sleep is classified in four stages, beginning with drowsy sleep, when we first start to nod-off, followed by stage 2, which features bursts of brain activity called ‘sleep spindles’.
Stages 3 and 4 of non-REM sleep are often referred to as deep sleep. Our brains exhibit slow delta waves and our bodies grow and repair. During REM sleep, the brain is more active, and experiments demonstrate that REM plays a vital role in learning and memory.
Do all sleep cycles look the same?
A healthy 7-hour sleep features an average of four to five 90 – 100-minute sleep cycles. However, there’s a popular but misguided idea that we can consider each sleep cycle as an independent unit. Some people propose that providing we achieve 28-35 sleep cycles each week, it doesn’t matter how these cycles are distributed.
Others suggest that we only need 4-5 hours of ‘core sleep’, which we can ‘top-up’ with naps. These ideas are appealing, particularly as a way to deal with irregular schedules and intensive work. Unfortunately, the evidence indicates that shortening sleep and considering sleep cycles independently is fraught with problems.
For example:
The proportion of non-REM and REM sleep in each cycle changes through the night.
We experience higher levels of non-REM sleep in earlier cycles, then increasing amounts of REM sleep in later cycles (1). Consequently, shortening sleep, and separating sleep cycles compromises REM, which can significantly impair learning and memory.
Some studies indicate that the negative impacts of dividing sleep into smaller chunks can be offset, following a period of adaptation.
However, participants in these studies followed a structured sleep schedule, in intense periods motivated by survival or competition, such as in single-handed sailing races (2). In contrast, most people who try to assemble sleep time from irregularly sized chunks end-up living in a continual state of jetlag, significantly increasing diabetes risk, and impairing cognitive performance.
Some research does indicate that 4.5-5.5 hours represents the duration of ‘core sleep’, which can satisfy the demands of job performance (2).
For time-poor high-performers, this is a tempting idea. However, is satisfactory performance enough? We know that repetitive work is less affected, but inadequate sleep impairs valuable capabilities such as problem-solving (3). Also, sufficiency for job performance does not satisfy the requirements for physical health.
Routinely sleeping for less than 7 hours:
- Harms metabolic health (4,5)
- Increases injury risk (6)
- Compromises the immune system (7)
- Is associated with decreased bone density (7).
We can’t treat sleep like a bank account
Sleeping four hours in one chunk, then three-hours in another is not equal to seven hours of continuous sleep. When we don’t sleep enough, we accumulate sleep debt at a very high-interest rate. While wearable devices only estimate sleep stages, they may provide an indication of your sleep duration and trends in the proportion of time spent in REM and non-REM sleep.
I encourage you to observe your sleep stages and see if you can notice any patterns related to sleep duration. But more importantly, prioritize adequate sleep. It’s one of the best investments you can make for sustainable high performance.
References
1. Lockley, S. & Foster, R. (2020) Sleep: A very short introduction. Oxford University Press. Oxford.
2. Claudio S. The Effects of Polyphasic and Ultrashort Sleep Schedules. In: Stampi C, editor. Why we nap. 1st ed. New York: Springer; 1992. p. 137–79.
3. Sio UN, Monaghan P, Ormerod T. Sleep on it, but only if it is difficult: Effects of sleep on problem solving. Mem Cogn. 2013;41(2):159–66.
4. Broussard JL, Ehrmann DA, Van Cauter E, Tasali E, Brady MJ. Impaired insulin signaling in human adipocytes after experimental sleep restriction: A randomised, crossover study. Ann Intern Med. 2012;157(8):549–57.
5. Shi SQ, Ansari TS, McGuinness OP, Wasserman DH, Johnson CH. Circadian disruption leads to insulin resistance and obesity. Curr Biol. 2013;23(5):372–81.
6. Johnston R, Cahalan R, Bonnett L, Maguire M, Glasgow P, Madigan S, et al. General health complaints and sleep associated with new injury within an endurance sporting population: A prospective study. J Sci Med Sport [Internet]. 2020;23(3):252–7. Available from: https://doi.org/10.1016/j.jsams.2019.10.013
7. Besedovsky L, Lange T, Born J. Sleep and immune function. Pflugers Arch Eur J Physiol. 2012;463(1):121–37.
8. Ochs-Balcom HM, Hovey KM, Andrews C, Cauley JA, Hale L, Li W, et al. Short Sleep Is Associated With Low Bone Mineral Density and Osteoporosis in the Women’s Health Initiative. J Bone Miner Res. 2020;35(2):261–8.