Considering that we spend a third of our time doing it, sleep is something of an evolutionary paradox. We need sleep to survive, and yet not only does it seem like a waste of time, it may also have left our early ancestors vulnerable to predators. Whatever the function of sleep, couldn’t we just do it whilst awake? After all, some animals, like dolphins and whales, achieve unihemispheric sleep – switching off just one side of the brain while the other stays alert – and horses take short naps during the day without needing to lie down.
That’s probably the wrong way to think about it, says Professor Derk-Jan Dijk, director of the Sleep Research Centre at the University of Surrey. “We sleep because the brain needs to do things that it can’t do when you’re awake,” he says. Another question is why you would always want to be awake, says Dijk. It’s energetically costly to be active, so you wouldn’t want to be awake longer than you need to. “From this viewpoint sleep becomes a period of adapted inactivity,” he says.
This would explain the variations in the animal kingdom. Horses need to be awake longer to meet their energy requirements from the relatively low-calorie foods they eat, but a tiger, having caught its high-calorie prey, would have plenty of time to sleep.
No species functions at its best over a 24-hour period, points out Russell Foster, professor of Circadian Neuroscience at the University of Oxford. Fantastic vision during the day is useless at night, so each species needed to make an “evolutionary decision” about when to take a break from normal activity and how to allocate resources accordingly. “You have all these essential housekeeping functions which you allocate to the appropriate phase of the rest-activity cycle, and that’s what sleep is,” says Foster. “It’s a multifunction state in which areas of the brain are more active, and huge bouts of absolutely critical functions are performed. That’s why we fall apart so rapidly if we don’t get sleep.” Some of these activities are physiological – the release of certain hormones, for example, which allow tissues to grow and repair.
More intriguing is the role of sleep in building memories, which has been understood since the 1880s, says Monika Schönauer of the Munich Centre for Neuroscience. Early psychological research showed that we forget less when we sleep than when we stay awake. We now know that sleeping after learning can help with both “declarative” memory tasks – like learning facts or vocabulary – as well as “procedural” or “implicit” memory tasks – like playing the piano or riding a bike.
When it comes to declarative memory, Schönauer and her colleagues have shown that sleeping after learning not only helps us set memories, but also retain them longer. The idea is that sleep protects memory formation from interference by other information. Her team compared the effect of sleep to other activities, such as meditation, which should also reduce interference to a minimum. A study published earlier this year compared sleep, wakefulness and meditation after learning. “We found that only sleep was beneficial for retention of different kinds of new information,” Schönauer says.
Several studies suggest that sleep helps us remember because the brain circuits involved in learning are reactivated while we slumber. Some of the most compelling evidence comes from experiments in which cues associated with the learning process are presented to sleeping subjects. In one key study, Björn Rasch at the University of Lübeck showed that odours helped people get a boost in declarative memory (in this case, learning the order of a deck of cards). The odours seem to act as a cue that reactivates patterns of brain activity that took place while participants learned the task.
Schönauer’s team have now shown that these effects extend to implicit learning, too – in this case, playing the piano. In a study published last year, participants learned to play a brief melody and then either slept or stayed awake for three hours. During that time they were re-played part of the melody. “Only the participants who slept improved their playing skills, making fewer errors in the half that we reactivated externally during the time they slept,” she says.
The role of dreams
Schönauer and others have begun to probe the idea that dreams too could play a role in memory and learning. In one experiment, she asked participants to listen to audio books before they went to sleep. Ninety minutes later they were awakened and asked if they had had any dreams and how much of the book they could remember. Independent researchers assessed the dream content to see if they could determine which audio book the person had listened to. The more information about the audio book could be detected in the dream content, they found, the more participants remembered from the audio book.
The connection between memory and sleep is also implicated in the idea that one role of sleep is to clear unwanted toxins from the brain. Most waste products are cleared from the body by the lymphatic system, but lymph is blocked from the brain by the bloodbrain barrier. In 2012, researchers identified a similar system in the brain: the glymphatic system run by glial cells, which support and protect brain cells. The following year, animal studies showed that the glymphatic system was especially active during sleep. “This system has received a lot of attention because research has linked it to neurodegeneration,” says Dijk. A study published earlier this year looked at the brains of a small group of patients with Alzheimer’s disease and found that poor sleep was associated with a build-up of amyloid plaques in the brain, as well as poorer memory function. “The general idea is that good sleep is a contributor to good brain health,” says Dijk. “It fits with a growing understanding that the symptoms of neurological diseases such as Alzheimer’s and Parkinson’s are closely associated with sleep, and seem to worsen with a lack of it,” he says.