Sleep · In-depth guide
Timing of sleep
The most important of these is the regularity of the waking time.
Key points
- A regular time of going to bed, going to sleep, waking up and getting up in the morning is an important factor in stabilizing and synchronizing the circadian rhythms.
- Sleep in adults is usually taken as a single episode at night (monophasic pattern), but in 85% of mammals, particularly those with a small body mass, sleep is polyphasic.
- Most people have a tendency to be more alert early in the morning soon after waking (morning types, larks) or late at night prior to falling asleep (evening types, owls).
- Daytime naps are normal in young children and common in the elderly and are taken especially at between 2.00 and 4.00 pmwhen the circadian rhythms favour sleep.
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Regularity of sleep
A regular time of going to bed, going to sleep, waking up and getting up in the morning is an important factor in stabilizing and synchronizing the circadian rhythms. The most important of these is the regularity of the waking time. The exact times that are adopted vary according to, for instance, age, social constraints such as work patterns, and the individual tendency to be a long or short sleeper, or a 'lark' or 'morning type' or an 'owl' or 'evening type'.
Regular sleep patterns ensure that the homeostatic drive to sleep is strong because they allow a sufficient interval after the previous main sleep episode. In the elderly especially, restricting the nocturnal sleep phase to 6-7h is usually preferable to sleeping for longer and often improves insomnia.
Monophasic and polyphasic sleep patterns
Sleep in adults is usually taken as a single episode at night (monophasic pattern), but in 85% of mammals, particularly those with a small body mass, sleep is polyphasic. A single prolonged sleep episode may expose these animals to more danger. A monophasic pattern enables the sleep debt accumulated during the waking period to be fully discharged whereas a polyphasic routine pays back this sleep debt in smaller units.
There is evidence that hunter-gatherer communities adopt a polyphasic sleep pattern with at any one time around 25% of the population awake at night and 10% sleeping during the day. A problem with this sleep pattern is that there are repetitive episodes of sleep inertia if naps last for more than 20 min, and particularly if they are longer than 60 min. The polyphasic pattern does, however, reduce the exposure to danger, and can enable a broader division of labour in the society.
Although a monophasic pattern is usual in adults, children obtain polyphasic sleep, usually with around 3-5h during the day at the age of 6 months and 2h during the day at 2 years. Initially these sleep episodes are almost random, but once the circadian rhythms mature at 3-6 months, more sleep is obtained at night than during the day. The polyphasic pattern is often re-entered in the elderly who tend to nap during the daytime, but occasionally it is retained throughout life. Leonardo da Vinci, for instance, is said to have slept for 15min every 4 h.
A polyphasic sleep pattern lessens the performance loss during sleep deprivation. The frequent naps repay the sleep debt exponentially. The deeper NREM sleep at the start of each nap is more time-effective than a more prolonged sleep episode with relatively more lighter NREM and REM sleep. Repayment of a sleep debt is facilitated if the naps coincide with an increased circadian or adaptive drive to fall asleep.
