Birds are the only non-mammalian taxonomic group to exhibit high amplitude slow oscillatory EEG activity during SWS and low amplitude mixed frequency EEG activity during REM sleep. Compared with human sleep, sleep in birds is quite fragmented, occurring in short bouts of 1–4 min duration. Periods of SWS last around 50 s in the beginning of the sleep period and decrease to ∼25 s at the end of the night. The average increase in SWA from wake to SWS is smaller than in mammals. While birds spend most time in SWS, REM sleep accounts typically for less than 10% of total sleep time and is distributed across sleep in relatively short epochs. Unlike mammals with the exception of whales and dolphins, birds regularly display unihemispheric sleep with one eye open, especially in threatening environmental conditions. Under safe conditions they prefer SWS with both eyes closed, suggesting that SWS expanding over the whole brain represents the more effective state. Consistent with this view, chickens spend more time sleeping with both eyes closed when they were deprived of sleep during the prior day. Whereas SWS can occur unihemispherically REM sleep does not.
In birds, like in humans, SWA primarily reflects the homeostatic regulation of sleep. SWA is higher after longer periods of wakefulness and highest at the beginning of the night declining exponentially across the sleep period. Sleep deprivation in pigeons and white-crowned sparrows produced compensatory increases in SWA during subsequent sleep. Comparable with conditions in mammals, SWA in birds seems to be additionally locally regulated, increasing specifically in brain regions that were used more extensively during prior wakefulness. Pigeons after watching David Attenborough's ‘The Life of Birds’ with only one eye, showed increased SWA (i.e., power in the 0.5–4.5 Hz frequency band) in the primary visual processing area (the hyperpallium) of the corresponding eye during subsequent sleep. The homeostatic regulation of SWA in birds, like that of humans, has been linked to an underlying process of synaptic down-scaling and renormalization that serves to globally balance synaptic connectivity, and thereby prepares the neuronal network for encoding of new information during the upcoming wake phase. In contrast to SWS, REM sleep in birds, like in mammals, appears to be primarily driven by the circadian rhythm and typically increases across nocturnal sleep, thus pointing to a differential function of both core sleep stages also in birds. However, there are observations suggesting that sleep deprivation in birds can be followed by increases in REM sleep as well which would implicate the existence of additional homeostatic mechanisms also for REM sleep regulation.
Because reptiles and amphibians lack distinct forms of SWS and REM sleep stages, it can be assumed that SWS in birds coevolved independently from mammals (Fig. 3a; Rattenborg, 2006). Indeed, the convergent evolution of this prominent feature of sleep might bear the answer as to the function of this sleep stage, which is possibly related to the comparably large and strongly interconnected brain in both mammals and birds.