In Search of Sleep An exploration of methods used to study the nature and function of sleep
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he average person will spend 36% of their life asleep, meaning that if you live until 90 you will have been asleep for the equivalent of 32.5 years. Despite this, the scientific community knows very little about what sleep actually is, and even less about why it occurs. There are two main schools of thought regarding the function of sleep: the universal function theory and the adaptive inactivity hypothesis. Some argue that sleep is a maladaptive state since it makes animals vulnerable to predation and is incompatible with feeding and mating. Yet sleep is ubiquitous across animals, so they argue that there must be a universal function that is so important to an animal’s survival that it outweighs the costs of sleeping. However, this fundamental function remains elusive - perhaps memory consolidation? Or regeneration of neurons? Others hypothesise that sleep itself can be seen as an adaptive state since it benefits animals by supressing activity at times of maximal predation risk and minimal opportunity for feeding, and permitting it during times of plenty. However, these theories don’t have to be mutually exclusive. Many tasks are likely to be carried out during sleep, but this doesn’t mean sleep evolved specifically for them to occur. Instead, sleep as a period of adaptive inactivity may have been co-opted by functions such as memory consolidation. There are several approaches one could take to unravel this quandary. These include focusing on the neuroscience of sleep, or comparative ecological studies. Past studies on the function of sleep have arguably produced more controversy than conclusions. However, current technological developments are now generating breakthroughs in knowledge. What is Sleep? irst, let us consider what we do know about sleep. Sleep is regulated by circadian rhythms and a homeostatic mechanism known as sleep pressure. It can be behaviourally defined as a rapidly reversible period of immobility, characterised by an increased arousal threshold and a period of compensato-
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Above and right Zubida Mukhtar
“Why do big brown bats sleep for 19 hours per day, whilst elephants sleep for just 2 hours?” ry sleep following sleep deprivation. By using probes to measure the electrical activity across the brain during wake and sleep one can see that there are several different types of sleep, known as Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM). REM is difficult to distinguish from wake, debunking the common misconception that our brains rest during sleep. NREM, on the other hand, is associated with high-amplitude, low-frequency waves of brain activity. Neuroscience Approach euroscience is the study of the nervous system and brain. Several recent breakthroughs in the neurobiology of sleep and circadian rhythms have stemmed from remarkable technological developments which have allowed for beautiful manipulations of neurons in animal brains. Optogenetics, which originated in Oxford in the humble fruit fly, and chemogenetics are two such tech-
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