In Search of Sleep

An exploration of methods used to study the nature and function of sleep

The 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 scientifc 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 benefts 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 specifcally 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? F 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 defned as a rapidly reversible period of immobility, characterised by an increased arousal threshold and a period of compensatory 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 diferent types of sleep, known as Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM). REM is difcult 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 N 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 fy, and chemogenetics are two such tech-

“Why do big brown bats sleep for 19 hours per day, whilst elephants sleep for just 2 hours?”

Above and right Zubida Mukhtar

niques which enable the remote controlling of neurons using light and specifc chemicals, respectively.

Nerve cells are activated by the movement of charged particles called ions. If enough ions of the appropriate charge move in and out of the cell, they trigger an electrical signal (an action potential). Optogenetics and chemogenetics exploit this mechanism. Specifc neurons of interest are genetically altered so that they express ion channels in their membranes that can be controlled remotely using light or specifc chemicals. Thus, neurons which researchers think may be involved in regulating sleep can be activated and silenced remotely so that one can witness the direct behavioural efects of these nerve circuits. This technique, although requiring refnement, is generating exciting research right here at Oxford University: in the Centre for Neural Circuits and Behaviour, sleep neurons have been identifed in Drosophila that result in sleep when activated. Comparative Approaches A n alternative approach to studying sleep is from an ecological and evolutionary perspective. All animals studied to date exhibit some form of sleep behaviour. However, this sleep comes in all shapes and sizes with no obvious pattern. Why do big brown bats sleep for 19 hours per day, whilst elephants sleep for just 2 hours? Why does sleep occur across both hemispheres of our brain simultaneously whilst in dolphins sleep occurs in only one at a time? These more unusual sleepers were often left out of traditional studies, as “anomalies”, but by studying them we gain an understanding of the selection pressures present and hopefully clues to the adaptive function of sleep.

Body mass has long been considered to have the strongest correlation with sleep duration, however, this is only signifcant in herbivorous terrestrial mammals. Despite similar genetics and cognitive abilities, closely related mammal species often have very diferent sleep durations. Researchers are now exploring the possibility that variables such as diet, habitat and social structure have a greater infuence on sleep duration than genetics. Brown bats eat mosquitoes and moths which are only active from dusk until early evening, and the 4 to 5 hours that the bat is active is synchronised perfectly to the active period of their prey. Meanwhile, elephants occupy a low trophic position, consuming low-calorie vegetation, and therefore must devote many hours to eating, which may explain their short sleep duration. This demonstrates that ecological variables may underpin the patterns in sleep, although statistical techniques are needed to test for the signifcance of these relationships.

Recording brain activity of wild organisms in their natural habitat would provide extraordinary insights into the adaptive function of sleep, suggesting that looking forward, the gold standard for studying sleep lies at the intersection of these two approaches. Laura Steel is studying for a PhD in Interdisciplinary Biosciences (Neuroscience & Animal Behaviour) at Magdalen College.