Why the Brain Needs the Night
For most of human history, sleep was regarded as a kind of temporary absence: the body rested, the mind switched off, and nothing of much importance happened until morning. This view began to change in the middle of the twentieth century, when researchers started to record the electrical activity of the brain through the night using sensors attached to the scalp. The recordings showed that the sleeping brain was far from idle. At certain points, in fact, it was almost as active as it is during the day, and the patterns of activity changed in a regular and predictable way.
We now know that a night's sleep is made up of a series of cycles, each lasting about 90 minutes. Within each cycle, the sleeper moves from light sleep into deep, slow-wave sleep and then into rapid eye movement (REM) sleep, the stage most closely associated with vivid dreaming. The balance between these stages shifts as the night goes on. Most deep sleep occurs in the first half of the night, whereas REM periods become longer towards morning. One consequence is that a person who goes to bed at the usual time but wakes two hours early loses a disproportionate share of REM sleep, even though the total reduction may seem modest.
One of the clearest functions of sleep concerns memory. In a series of experiments, the psychologist Ingrid Solberg asked volunteers to learn routes through an unfamiliar virtual town. Half learned the routes in the evening and were tested after a night's sleep; the others learned them in the morning and were tested twelve hours later, without having slept. The first group recalled significantly more of the routes. Solberg argues that sleep does not simply protect new memories from interference but actively strengthens them, replaying recent experience and linking it to what the brain already knows.
Sleep also appears to play a role in keeping the brain physically healthy. The neuroscientist Tomas Weller has studied the fluid that surrounds brain cells and found that, during deep sleep, the spaces between cells widen, allowing this fluid to flow more freely. As it does so, it carries away waste proteins that build up during waking hours, some of which are thought to contribute to disease if they accumulate over many years. In Weller's words, the brain uses the night for housekeeping that it is simply too busy to carry out during the day.
When we sleep is governed largely by an internal body clock, which is set each day by exposure to daylight. This clock does not stay the same throughout life. During adolescence, it shifts later, so that teenagers naturally feel sleepy later at night and find it hard to wake early. The sleep researcher Amara Osei points out that this creates a conflict with school timetables, which often require older pupils to be in class at an hour when their bodies are still programmed for sleep. She has called for secondary schools to begin lessons later in the morning, citing trials in which attendance and concentration improved when the start of the school day was delayed by even half an hour.
The effects of losing sleep are well documented. After a single short night, attention begins to lapse, reaction times slow and people become more irritable. Over longer periods, poor sleep has been linked to changes in appetite, weakened immunity and a greater risk of accidents. Perhaps the most troubling finding comes from the work of Henrik Dahl, who kept volunteers on six hours of sleep a night for two weeks. Their performance on attention tests declined steadily throughout the study, yet when asked how they felt, most reported that they had adjusted and were functioning normally. Dahl concludes that tired people are poor judges of their own impairment.
This may explain why so many people claim to need little sleep. There do appear to be individuals who function well on five or six hours, and some of them share particular genetic variations. However, such people seem to be extremely rare. For the great majority of those who say they manage perfectly well on very little rest, the more likely explanation is that they have become so accustomed to feeling tired that they no longer notice it.
What, then, can people do to sleep better? Researchers generally agree that keeping a consistent waking time, even at weekends, helps to stabilise the body clock, and that bright light early in the day makes it easier to fall asleep at night. Naps are more controversial. The physiologist Rafael Moreno believes that a short rest of around twenty minutes in the early afternoon can restore alertness, but he warns that longer naps allow the sleeper to enter deep sleep, from which waking is difficult and which can leave people feeling groggy for some time afterwards.
