It is important to realise the distinction between REM and NREM for energy conservation. The energy consumption of the brain drops only in NREM sleep; during REM the brain is still relatively active. This leads to suggest that it is only NREM sleep that has evolved for energy conservation. It is also argued that animals that are more primitive only have NREM sleep. REM sleep appears to have only evolved about 50 million years ago in mammals. It may be that NREM sleep evolved first for energy conservation, whereas REM sleep may have evolved later to maintain brain activity. This is supported by the greater need for REM sleep in infants with developing brains.
Another evolutionary explanation states sleep is determined by food requirement. Grazing animals such as horses eat food that is poor in nutrients, so spend most of their time eating, and little time sleeping. Predatory animals eat food high in nutrients so don’t need to eat much and so can sleep longer and conserve more energy.
Another factor affecting sleep is whether an animal is predator or prey. Predators can sleep longer due to a low risk of being killed, but prey is vulnerable when sleeping, so sleep less. Sleep must therefore serve a vital function, as if it did not, natural selection would have evolved it out.
Capellini suggested that the energy conservation hypothesis may be wrong and the foraging and predator avoidance may be right. They argued that previous research methods were flawed because methods used to collect data were not standardised, and therefore comparison between species were meaningless and had low internal validity. When conducting a study using only standardised procedures they found that the relationship between predation and sleep was complex and unexplained; some animals were exposed and slept less, but so did animals that slept protected by others. This suggests that sleep may be biologically necessary and shape by the pressures of each species’ environment around them. However, the lab setting may have affected the quality of sleep and so lacks external validity meaning the results are not generalisable to the wider world.
Allison found that prey species did turn in sleep less, but again there were exceptions such as rabbits that slept just as much as moles, which had a low danger rating. This then appears to go against the predator avoidance theories and the evolutionary approach. It implies that behaviour such as sleep may not be simply evolutionary, but biologically essential for bodily functions highlighting the role nature plays rather than sleep having evolved due to nurture and environmental pressures.
Meddis proposed the waste of time hypothesis, suggesting sleep serves the purpose of helping animals avoid predators when they may be most vulnerable by sleeping in darkness and hidden. It endures they stay still when having nothing better to do. Siegal concurred with this view, arguing being away would be riskier due to exposure. The only possible explanation could be sleep enables energy conservation and avoiding dangers.
However, the evolutionary approach fails to address some key aspects of sleep, such as why we long for sleep when sleep deprived. Horne suggests a solution in a combined approach. He suggested a distinction between core and optional sleep. Core sleep being SW, that is required for body and brain processes, optional sleep is REM and is dispensable like conserving energy. Restoration theorists would argue against this and city the link between REM and brain development, suggesting even REM serves a purpose beyond simply wasting time.