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What is the purpose of sleep?

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Almost all vertebrates require sleep, to a greater or lesser extent. Studies show that animals deprived of sleep can die sooner than those deprived of food, and sleep is thought to be an essential metabolic requirement.

An organism’s metabolism has two phases. In the catabolic phase, during wakefulness, energy is expended carrying out bodily functions, such as feeding and mating, processes that can cause oxidative stress damage to the organism’s cells. During sleep, metabolism enters the anabolic phase, with reduced heart and lung rhythm and general muscle relaxation. It is during this phase that the organism grows and heals.

In humans, some effects of sleep deprivation on mental health are well known. Even mild or moderate deprivation can affect concentration and learning ability, and cause symptoms of stress. Research in rats has shown that lack of sleep can increase the levels of the stress hormone cortisol in the brain, leading to damage of the hippocampus, which is responsible for maintaining memory.

Other studies have shown a correlation between reduced sleeping times and increased body mass index (BMI), through disturbance of the levels of two key hormones involved in appetite regulation —  leptin, which suppresses appetite, and ghrelin, which stimulates it. Individuals with reduced sleep levels showed a drop in leptin levels, but an increase in levels of ghrelin, with a corresponding increased BMI.

Researchers at the University of California at Los Angeles have questioned the purpose of sleep in animals, citing examples of species that can do without sleep for long periods. For instance, newborn dolphins and their mothers hardly sleep at all for several weeks after birth, when these animals normally migrate.  If sleep has a universal function, argue the researchers, how can certain species survive for long periods without it? They propose that the main function of sleep is to allow animals to adapt to the world around them, by optimising energy expenditure and affording themselves protection by avoiding predators.  But other researchers are sceptical, saying that rest alone would provide similar protection.

Whatever the answer, there can be no doubt that there is something particularly comforting about the prospect of a good night’s sleep; a lie down just does not compare.

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