Read more: 鈥�Sleep and dreaming: The how, where and why鈥�
WE SPEND about a third of our life doing it. If deprived of it for too long we get physically ill. So it鈥檚 puzzling that we still don鈥檛 really know why it is that we sleep.
On the face of it the answer seems obvious: so that our brains and bodies can rest and recuperate. But why not rest while conscious, so that we can also watch out for threats? And if recuperation means things are being repaired, why can鈥檛 that take place while we are awake?
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Scientists who study how animals eat, learn or mate are unburdened by questions about the purpose of these activities. But for sleep researchers the big 鈥淲hy?鈥� is maddeningly mysterious.
Sleep is such a widespread phenomenon that it must be doing something useful. Even fruit flies and nematode worms experience periods of inactivity from which they are less easily roused, suggesting sleep is a requirement of the simplest of animals.
But surveying the animal kingdom reveals no clear correlation between sleep habits and some obvious physiological need. In fact there is bewildering diversity in sleep patterns.
Some bats spend 20 hours a day slumbering, while large grazing mammals tend to sleep for less than 4 hours a day. Horses, for instance, take naps on their feet for a few minutes at a time, totalling only about 3 hours daily. In some dolphins and whales, newborns and their mothers stay awake for the entire month following birth.
鈥淪ome newborn dolphins and their mothers stay awake for the entire month after birth鈥�
All this variation is vexing to those hoping to discover a single, universal function of sleep. 鈥淏odily changes in sleep vary tremendously across species,鈥� says Marcos Frank at the University of Pennsylvania in Philadelphia. 鈥淏ut in all animals studied so far, the [brain] is always affected by sleep.鈥�
So most sleep researchers now focus on the brain. The most obvious feature of sleep, after all, is that consciousness is either lost, or at least, in some animals, reduced. And lack of sleep leads to cognitive decline, not only in humans, but also rats, fruit flies and pretty much every other species studied.
Much of our slumber is spent in slow-wave sleep, also known as stage 3 or deep sleep (see diagram), during which there are easily detectable waves of electrical activity across the whole brain, caused by neurons firing in synchrony about once a second. This is interspersed with other phases, including rapid-eye-movement sleep, where brain activity resembles that seen during wakefulness, and transitional stages between the two states.
It is slow-wave sleep that is generally thought to do whatever it is that sleep actually does. As well as appearing to be the most different to the brain鈥檚 waking activity, the waves are larger at the beginning of sleep, when sleep need is presumably greatest, and then gradually reduce. And if you go without sleep for longer than usual, these slow waves are larger when you do eventually nod off.
Explanations for sleep fall into two broad groups: those related to brain repair or maintenance, and those in which the sleeping brain is thought to perform some unique, active function. There has been speculation over the maintenance angle for over a century. It was once a fashionable idea that some kind of toxin built up in the brain during our waking hours which, when it reached a certain level, made sleep irresistible. Such a substance has never been found, but a modern version of the maintenance hypothesis says that during the day we deplete supplies of large molecules essential for the operation of the brain, including proteins, RNA and cholesterol, and that these are . It has been found in animals that production of such macromolecules increases during slow-wave sleep, although critics point out that the figures show a mere correlation, not that levels of these molecules control sleep.
The unique function school of thought also has a long pedigree. Sigmund Freud proposed that the purpose of sleep was wish fulfilment during dreaming, although scientific support for this notion failed to materialise.
There is good evidence, however, for sleep mediating a different kind of brain function 鈥� memory consolidation. Memories are not written in stone the instant an event is experienced. Instead, initially labile traces are held as short-term memories, before the most relevant aspects of the experience are transferred to long-term storage.
Action replay
Several kinds of experiment, in animals and people, show that stronger memories form when sleep takes place between learning and recall. Some of the most compelling for this idea came when electrodes placed into rats鈥� brains showed small clusters of neurons 鈥渞eplaying鈥� patterns of activity during sleep that had first been generated while the rats had been awake and exploring. 鈥淢emory representations are reactivated during sleep,鈥� says Jan Born at the University of T眉bingen in Germany.
Many labs remain focused on how memory systems are updated during sleep, but since 2003 a new idea has been gaining traction. It straddles both categories of theory, concerned as it is with neuronal maintenance and memory processing.
The hypothesis concerns synapses, the junctions between neurons through which they communicate. We know that when we form new memories, the synapses of the neurons involved become stronger. The idea is that while awake we are constantly forming new memories and therefore strengthening synapses. But this strengthening cannot go on indefinitely: it would be too expensive in terms of energy, and eventually there would be no way of forming new memories as our synapses would become 鈥渕axed-out鈥�.
The proposed solution is slow-wave sleep. In the absence of any appreciable external input, the slow cycles of neuronal firing gradually lower synaptic strength across the board, while maintaining the relative differences in strength between synapses, so that new memories are retained (see diagram).
There is now much evidence to support what is known as the 鈥溾��. In humans, brain scans show that our grey matter uses more energy at the end of the waking day than at the start. Giulio Tononi and Chiara Cirelli of the University of Wisconsin-Madison, who proposed the hypothesis, have shown that in rodents and fruit flies, synaptic strength increases during wakefulness and falls during sleep. The pair have also shown that when people learn a task that uses a specific part of the brain, that part generates more intense slow waves during subsequent sleep. This kind of downscaling is best done 鈥渙ffline鈥�, says Tononi. 鈥淵ou can activate your brain in all kinds of ways, because you don鈥檛 need to behave or learn.鈥�
Synaptic homeostasis has not won over everyone, but it is certainly getting a great deal of attention. It is, says Jan Born, 鈥渃urrently the most influential [theory] among sleep researchers鈥�. Frank, however, would like Tononi and Cirelli to provide more detail about mechanisms.
Neither is Jerry Siegel convinced. A neuroscientist at the University of California, Los Angeles, Siegel is sticking with his provocative theory that sleep is simply an adaptive way of saving energy when not doing essential things, such as foraging or breeding, which are in fact more dangerous than napping someplace safe. For Siegel, sleep habits reflect the variety of animal lifestyles, with different species sleeping for different purposes.
It鈥檚 certainly possible that a phenomenon as complex as sleep performs a multitude of functions, agrees Jim Horne, who studies the impact of sleep loss on health at Loughborough University, UK. And, given the complexity of the human brain, our sleep may well be among the most complicated of all.
Perhaps then it should be no surprise that theories of sleep function are so diverse. Fathoming whether the big 鈥淲hy?鈥� of sleep will yield a single, succinct solution or require myriad answers is likely to keep biologists up at night for a little while yet.
