All living organisms, from bacteria, to plants, to animals, have a rhythmic cycle of activity and rest. These cyclic patterns result from an internal system that is independent, yet synchronized to the environment day/night cycle. Some organisms, such as humans, are diurnal and are active during the day period, whiles are nocturnal and are active during the night.
Sleep, in its most basic form is observed in all animals. However, the more complex sleep patterns, defined by characteristic behavioral and electrophysiological features, is observed in mammals and birds. This includes different sleep stages, including paradoxical sleep and slow wave sleep. In humans, this translates to the different sleep stages known as rapid eye movement (REM) sleep and the different stages of Non-REM sleep, respectively.
The study of sleep across different animal species (the phylogeny of sleep) may help in identifying the function(s) of sleep. As such, the main theory for the basis of a rest or sleep period is for restoring the body’s homeostatic state that gradually becomes un-balanced in wakefulness. Additionally, sleep may also play a role in metabolic regulation, as the complexity of sleep is further developed in endothermic animals, including mammals, birds and some fish, who internally regulate their body temperature. Lastly, sleep, and more specifically paradoxical or REM sleep, may be a result of the development of the brain.
Nevertheless, as sleep and rest are found, different extent, in all living creatures, it a basic and essential component of life.
- Tobler, I. Chapter 9 – Phylogeny of Sleep (2010). In Kryger, M., Roth, T., Dement. W. Principles and Practices of Sleep Medicine, fifth edition (pp. 112-125).
- Tobler, I. Chapitre 8 – Phylogenèse du sommeil (2012). In Billard, M. and Dauvilliers, Y. Les Troubles du Sommeil, 2nd Edition (pp. 83-91).