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Sleep is a ubiquitous behavior found in all animals studied to date from jellyfish to humans In mammals sleep can be divided into two major states non-rapid eye movement NREM sleep and REM sleep based on brain electrophysiological signatures and behavior Although important advances have been made in the past decades in our understanding of the mechanisms of sleep a consensus has not been reached regarding its functions In this respect comparative studies may not only give insights into the evolution of sleep but may reveal general principles applicable to many taxonomic groups Birds are particularly interesting for comparative studies because despite being evolutionarily more closely related to crocodiles and other non-avian reptiles than mammals they exhibit two sleep states remarkably similar to mammalian NREM and REM sleep Thus research on birds could complement and further our understanding of the evolution and function s of sleep In this manuscript I present data on three projects investigating behavioral physiological and neurophysiological correlates of sleep in birds with a focus on REM sleep First we recorded cortical brain temperature in sleeping birds to understand whether and how it varies with sleep states We found that as in mammals cortical temperature decreases during NREM sleep and increases during REM sleep However the rates of cooling and warming were slower than in mammals In a second study we investigated state dependent changes in pupil size in birds In mammals the size of the pupil reflects the state of the brain during wakefulness and sleep Surprisingly we found that state-dependent changes in pupil size in birds are opposite those described in mammals during both wakefulness NREM sleep and REM sleep Through pharmacological experiments we also showed that pupil constrictions in birds unlike mammals are mediated via nicotinic cholinergic receptors The opposite pupillary behavior is thus linked in part to the fact that in contrast to the mammalian iris which is controlled by smooth muscles the avian iris is controlled by striated muscles Finally in a third study we conducted for the first time fMRI recordings in sleeping birds and obtained state-dependent BOLD activity Although still in progress this data suggests that REM sleep is associated with broad activation of cortical and subcortical structures spanning visual and somatomotor pathways and the basal ganglia Unexpectedly NREM sleep was associated with enhanced activity in the ventricular system which might reflect enhanced activity of the glymphatic system when compared to REM sleep If correct these results suggest that the benefits of expressing REM sleep outweigh the costs of temporarily suspending the proposed clearance of metabolic waste products via glymphatic flow during NREM sleep Taken together our data show that different components of REM sleep manifest in various ways across taxonomic groups ranging from similar to opposite This variation serves as a rich resource for investigating the evolution and functions of REM sleep Notably shared features are likely linked to the core function of REM sleep whereas those that differ might reflect taxon-specific functions or even epiphenomenon resulting from the specific neurophysiology of an organism Through embracing this diversity researchers will likely obtain a more comprehensive understanding of what REM sleep is and does that could not be obtained through focusing exclusively on mammals

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New perspectives on avian REM sleep

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