The importance of deep sleep Sleep is divided into two distinct states, Rapid Eye Movement (REM) and Non-REM sleep, with Non-REM sleep being further divided into three stages; N1, N2, N3, each of increasing depth. During the night, you pass through the four sleep stages: N1, N2, N3, and REM sleep in what are known as ‘sleep cycles’. Sleep progress cyclically from N1 through to REM, then begins again with stage N1. Each sleep cycle lasts approximately 90 to 110 minutes in adults. The first couple of sleep cycles have long periods of uninterrupted deep N3, or Slow Wave Sleep (SWS), with relatively short REM periods. Later in the night, the REM periods lengthen and SWS is mostly absent. Thus, the first third of the night is predominantly SWS sleep and the latter part of the night is spent in the lighter stages, N1, N2, and REM sleep. Below is a stylised representation of a night’s sleep for an adult showing the distribution of the various sleep stages across the night. Stage N1 (1-5% of sleep) is the lightest stage of sleep and is the transition between wake and sleep. Stage N2, which accounts for 45-50% is known to play a part in memory but as yet we do not entirely understand why we spend half the night in this stage. As sleep becomes deeper, slow brain waves (called delta waves) start to appear, and we enter N3 or SWS (20-25% of sleep). N3 is the deepest stage of sleep and when someone is in SWS, it can be very difficult to wake them. SWS is believed to be most closely linked with the restorative processes of sleep and is thus the part of sleep that makes you feel like you have had a good sleep. It also plays a key role in making you feel well-rested and energetic during the day. (Groeger, J.A., Stanley, N., Deacon, S. and Dijk, D.J., 2014. Dissociating effects of global SWS disruption and healthy aging on waking performance and daytime sleepiness. Sleep, 37(6), pp.1127-1142.) SWS is essential for memory; during deep sleep, we lay down new memories based on what we experienced during the previous day, we also forget the unimportant information. SWS is also involved in learning new tasks as during deep sleep we rehearse the behaviour such that in the morning we can be approx. 17% better at the task. (Peigneux P, Laureys S, Delbeuck X, Maquet P. Sleeping brain learning brain. The role of sleep for memory systems. Neuroreport. 2001;12:A111–24) Because SWS is so vital to memory and learning it should be no surprise that children have proportionally more SWS than adults. Another reason that children have a lot of SWS is that it is the only time that you physically grow. It is during SWS that some people, particularly children, experience behaviours (known as parasomnias) such as bedwetting, sleep talking, sleepwalking or night terrors. Both short and long sleepers have the same amount of SWS; thus, it seems as though a minimum amount of N3 deep sleep is needed per night, no matter how long the total sleep time. Given the importance of SWS after partial or complete sleep deprivation, the brain attempts to make up all the missed SWS; this is why after a night of poor sleep the next night you sleep really well and deeply. (Ferrara, M., De Gennaro, L. and Bertini, M., 1999. Selective slow-wave sleep (SWS) deprivation and SWS rebound: do we need a fixed SWS amount per night. Sleep Res Online, 2(1), pp.15-9.) SWS is also when the pituitary gland secretes important hormones, like human growth hormone, (Van Cauter, E. and Plat, L., 1996. Physiology of growth hormone secretion during sleep. The Journal of paediatrics, 128(5), pp.S32- S37).leading to growth and development of the body and thus in adults it is important as it stimulates muscle growth and repair, bone building and fat burning, and therefore helps athletes recover. Studies have shown that sleep deprivation slows the release of growth hormone and this has a negative effect on the day to day recovery of leg strength/power, sprint performance and self-selected exercise intensities (pacing strategies). Sleep loss also retards the repletion of muscle glycogen and this may additionally contribute to a decline in athletic performance. (Caia, J., Kelly, V.G. and Halson, S.L., 2018. The role of sleep in maximising performance in elite athletes. Sport, recovery, and performance: Interdisciplinary insights, pp.151-167.) It is during deep sleep that our brain's ability to efficiently wash away waste and toxic proteins. Over the day, toxic proteins such as beta-amyloid and tau accumulate in the brain and are removed at night during deep sleep by the action of the glymphatic system. Recent research has shown that during deep sleep, large oscillations of Cerebral Spinal Fluid appeared every 20 seconds, and these washed over the brain flushing out the toxic brain proteins. The action of the glymphatic system and the effect of disturbed sleep of levels of beta-amyloid and tau may explain the link between ageing, sleep deprivation, and heightened risk for Alzheimer's disease. (Grubb, S. and Lauritzen, M., 2019. Deep sleep drives brain fluid oscillations. Science, 366(6465), pp.572-573.) One of the most common sayings about sleep is "One hour's sleep before midnight is worth two after,” (the earliest source, from 1640, gives the idea that one hours sleep before midnight is worth three after but from 1670 the proverb equates its worth to two hours). This proverb is simply explained by the fact that deep SWS is predominant in the first third of the night and so much of the restorative benefit of sleep is achieved in that period. Thus, given a bedtime of 9 or 10 o’clock, a person gets most of his deep, restful sleep in the hours before midnight with lighter and less refreshing sleep in the hours after midnight. So, it has nothing to do with ‘midnight’ per se and more to do with the timing of deep restorative sleep occurring in the first third of the night, whenever this occurs.