A theory that explains the functions of sleep is the evolutionary theory. This theories main assumption is that sleep must be adaptive due to the idea that it is helping the particular species survival if it adapts to the new circumstance or environment.
The main researcher into the evolutionary theory was Meddis, he assumed that animals sleep to keep safe. He stated that the sleep to keep out of danger from predators when they are not able to be active. This is suggesting that it is an advantage for the animals to be still when it is too dark to see anything, including food or threats. Meddis acknowledged that predators can sleep for longer because they have no threat to attack them even when it is light, however in contrast prey should sleep when they are least vulnerable (e,g. In the dark) because then they are not in any true danger. Meddis finally said that sleep is not only for keeping safe but something for the animals to do when they have nothing else to do (e.g. mating, feeding).
A criticism of this explanation is that Meddis does not address the issue that, if safety from predators is so key, them we surely we would have be alert all the time. Being asleep puts the animal in more danger as it is unaware of its surroundings, this means it is more vulnerable to predators which contradicts what Meddis proposes. therefore surely it would of made sense that they evolved to be awake 24/7 which would mean they were always aware of their surroundings and situation. Bentely supported this point as he stated that if being unnoticeable was the great benefit and survival tactic of sleep, then it is very confusing as to why so many humans sleep noisily and attract so much attention (e.g. snoring), Meddis once again failed to respond to this point showing this is a flaw in his theory.
Another criticism of Meddis` theory is that he fails to explain why there are different stages of sleep. If sleep is so simply just `to keep animals safe` then why is sleep been proven to be so complicated. Sleep has several stages, and two types (REM and NREM), however Meddis ignores this and he states it is for a much simpler reason, therefore this makes this theory reductionist.
On the other hand a strength to this theory is that it is supportive evidence. Lesku et al conducted an observational research looking at the sleep patterns in 50 different species. They found a positive correlation that predators (carnivores) sleep more than prey (herbivores) as they are safer, carnivores sleep longer than herbivores as their food sources are not rich enough in key nutrients whereas carnivores can eat every so often but they eat larger amounts of high protein foods. They also found that those animals in which have a safer place to sleep, sleep longer than animals with dangerous habitats, this is because they are more relaxed and have less to be cautious about. However these correlations only show association but we can not form a causal relationship, so they do not explain what is the cause or effect of the animals not being able to sleep. Also, most studies are set in a zoo environment therefore the internal validity would be lowered as the ecological validity is low and therefore the settings cannot be generalised.
In conclusion, Meddis`s evolutionary theory can be criticised because it fails to explain sleep is complicated and also it does not respond to the point that if sleep was so dangerous we would have evolved to be awake all the time. However the theory can be supported by researcher that shows those with a safer sleeping environment sleep for longer.
Another main researcher who studied the evolutionary theory is Webb who completely contradicts Meddis as he states that sleep is purely to conserve energy.
Webb introduces the idea that when animals cannot be active, sleeping conserves their energy because they are not burning