Why we sleep: the evolutionary pathway to the mammalian sleep

Area of Science:

• Neuroscience
• Evolutionary Biology
• Sleep Science

Background:

• Defining the cause of sleep requires clarifying causal relationships and distinguishing behavioral from polygraphic sleep.
• Sleep's evolutionary origins are explored through vertebrate brain evolution, from simple nerve cords to complex mammalian and avian structures.

Purpose of the Study:

• To clarify the definition of sleep and its causal relationships.
• To investigate the phylogenetic origins of mammalian and avian sleep through brain evolution.

Main Methods:

• Review of existing literature on sleep, causality, and vertebrate brain evolution.
• Comparative analysis of brain structures and their relation to waking and sleep patterns across species.

Main Results:

• Vertebrate brain evolution led to distinct waking types: diurnal (poikilotherm brainstem-controlled) and nocturnal (mammalian thalamocortical).
• Mammalian sleep, particularly slow-wave sleep, likely evolved from suppressed early waking systems.
• Rapid eye movement sleep in mammals may stem from the nocturnal rest patterns of poikilotherms.

Conclusions:

• Mammalian sleep architecture is an evolutionary remnant, with cortical waking being the primary mammalian innovation.
• Understanding sleep's evolutionary history provides insights into its fundamental biological purpose and mechanisms.