New insights into our sleep requirements – from the amount we need to when, where and how we do it – have been supplied by recent research. The bottom line is that the quality of sleep varies from person to person and also changes throughout life
How much is enough?
A newborn baby may sleep as much as 18 hours a day, while a middle-aged executive may manage on as few as 5 hours. But how much is healthy and how much do we need? The short answer is that there is no answer: your needs depend on your age and gender, and it varies between individuals.
Young animals generally sleep more than adults and humans are no exception. The structure and intensity of sleep is different too. In young people there is a preponderance of REM sleep, and non-REM sleep is very deep, probably to aid brain maturation. Deep sleep and REM sleep both contribute to plasticity of neuronal networks, which could help with the acquisition of new skills. A recent study found babies sleep more during growth spurts ().
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During adolescence sleep becomes shallower and shifts to later hours, reflecting extensive brain rewiring. The frontal lobe – responsible for executive functions such as planning and inhibiting inappropriate behaviour – shows a marked fall in synapse density as the result of neuronal pruning. Teenagers are not just being lazy when they don’t want to get out of bed. Their adolescent biology may also prefer an adjustment of school hours.
There is good evidence that young people don’t get enough sleep. When they live on an 8-hour sleep schedule they remain sleepy, and much more so than older people on the same schedule (). If young adults are forced to stay in bed in darkness for 16 hours a day they initially sleep for as long as 12 hours. However, after several days they level off to just under 9 hours, showing that they were paying off a sleep debt ().
As we age we sleep less and spend less time in both deep sleep and REM sleep. At the same time, learning new skills proves to be more difficult. One fascinating question is whether, by preventing age-related changes in sleep, we can halt the age-related decline in mental dexterity.
If older volunteers are forced to stay in bed in darkness, they tend to sleep for around 7.5 hours. Studies of those who live longest find they report sleeping between 6 and 7.5 hours a night ().
Gender differences also occur in sleep and circadian rhythms. Such differences have been seen in cats, rodents, fruit flies and humans. Women enjoy a greater quantity of deep sleep and sleep longer. There are gender differences in the circadian clock too: the period of the clock is 6 minutes shorter in women than in men and this may explain why on average women go to bed earlier, wake up earlier and are more likely to rate themselves as “morning types” than men ().
What controls sleep?
The human body and many of its functions are synchronised with a 24-hour cycle via numerous biological clocks. For instance, brain activity while we are awake is largely dependent on the time of day (, vol 114, p 1047). The circadian timing system is made up of a core set of genes, the protein products of which act in interconnected feedback loops to precisely regulate their own production over a 24-hour cycle. Although the “central” biological clock, located in the brain, largely dictates wakefulness and sleep, other factors come into play.
A key factor is light (see diagram). We all know it is difficult to fall asleep in a brightly lit room. This is because light stimulates specialised cells in the retina, called intrinsically photosensitive retinal ganglion cells, which synchronise the biological clock to the day-night cycle and stimulate other brain areas involved in alertness. Even ordinary room light, or the light from a computer screen (, vol 110, p 1432), can influence the clock and suppress secretion of the sleep-inducing hormone melatonin (, vol 96, p E463).
A recent study carried out at the Surrey Sleep Research Centre in Guildford, UK, found that reducing the intensity of evening light, and/or using a light with less blue and more yellow, minimises the disruptive effect on sleep (). This suggests that it is possible to improve sleep by developing better evening lighting and being more aware of the disruptive effects of artificial light.
“It is possible to improve sleep by developing better evening lighting and being more aware of the effects of artificial light”
However, even in bright sunlight it can be difficult to stay awake if you have been active for a very long time. This “sleep pressure” is largely created by a neuromodulator called adenosine. Caffeine, the most widely used stimulant in the world, blocks the receptors where adenosine acts in the brain.
Bad day, good night
We often sleep well on holiday thanks to an absence of alarm clocks and work-related stress. Lab studies have indeed shown that worry means shallower sleep and more waking up. We also know that age has a profound effect on sleep quality. Older people are more susceptible to the sleep-disrupting effects of stress, caffeine and alcohol. For such people, a few drinks in the evening may severely disrupt sleep in the second half of the night.
Unsurprisingly, the best time to sleep is at night. If we attempt to sleep during the day without shifting our biological clock, we tend to slumber for between 1 and 3 hours less than at night.
Daytime sleep is also different in quality. During the night REM sleep increases as sleep progresses but during the day it often decreases. Sleep spindles, brainwaves that are characteristic of non-REM sleep and are implicated in memory consolidation, are more abundant at night. Other differences can be seen in the rest of the body. Sleep at night results in a lower body temperature and higher concentrations of the sleep hormone melatonin, whereas the reverse occurs during daytime sleep. These changes can be harmful to health.
Night shift workers often have disrupted daytime sleep and in the long run tend to have a higher risk of cardiovascular diseases and diabetes ().
