- The Evolutionary approach emphasizes the role of sleep in an animal's general lifestyle. it considers different characteristics of a species such as brain and body size, their habitat, whether they are herbivores or carnivores and much more.
- The Restoration approach emphasizes the possible restoration functions of sleep i.e. how it makes us feel rested and refreshed. The idea is that during sleep, our physiological systems can rest and recover from activities of the day fro example, restoring hormones, neurotransmitter levels and repairing damaged body tissues.
- one Advantage of both theories is that they aim to provide a general explanation for the function of sleep which can be applied to any species.
Sleep is found throughout the animal kingdom but it varies greatly across species in terms of amount and in some cases the patterning of Rapid Eye Movement(REM) Non-Rapid Eye Movement(NREM) sleep. Rem sleep tends to be the more deeper stages of sleep where the body becomes paralysed but the brain remains very much active, more vivd dreams occur at this stage. NREM sleep tends to be the less deeper stages of sleep and less less vivd dreams occur at this stage and we tend to have more of this per sleep cycle. the varied pattern of sleep helps us to identify some general relationships for example, large animals tend to sleep less than smaller animals; elephants and cows tend to sleep for about 4 hours a night while cats sleep for about 15 hours.
- Meddis(1975): put forward one of the first theories of sleep supporting the evolutionary approach. He proposed that the key variable was danger and that the simple purpose of sleep was to keep an animal inconspicuous and safe at times when active behaviour was impossible. For instance, diurnal animals are adapted to be active during the day and sleep at night and nocturnal animals such as Owls are adapted to be active during the night and keep themselves safe during the day by sleeping.
- Allison and Cicchetti(1976): conducted a study looking at the sleep characteristics in 39 animal species. They focused on whether animals were predators or preys and they found that there was a relationship between the level of danger experienced by an animal in its normal habitat and ts normal sleep time. these two variables were negatively corelllated as it was found that prey animals sept less than predators such as Lions and Leopards.
- Strengths: Allison and Cicchetti's study show support for Meddis's theory that the key variable that determines a species' sleep parttenr is danger and that our habitat is the most important because it determies the level of danger we are exposed to which in turn determines our sleep pattern. A strength of Meddis's theory is that it can be applied to humans too because it provides sound reasoning as to why humans are active during the day but asleep during the night; we have poor night vision therefore, we sleep during the night to lead ourselves…