Studies on sleep patterns and sleep homeostasis in birds: An ecological approach

Sleep is a complex phenomenon that consists of two completely different and alternating states, slow-wave sleep (SWS) and rapid-eye-movement sleep (REM sleep). Each of these two states is thought to play an important role in supporting brain and bodily functions. Yet, how exactly sleep fulfills these functions is a topic of ongoing research and debate. Most of what is known about sleep is derived from studies that were done in mammals under strictly controlled laboratory conditions. However, sleep is not restricted to mammals but is thought to be present in all living animals. Moreover, studies in a laboratory setting may not provide a complete picture of the regulatory processes and functions of sleep under natural conditions. For that reason, I measured sleep in three bird species under both laboratory conditions and semi-natural conditions: the European jackdaw (Coloeus monedula), the European starling (Sturnus vulgaris) and the barnacle goose (Branta leucopsis). The results provide evidence for homeostatic regulation of SWS in birds similar to what has been reported for mammals, but also produced unexpected findings. For example, the geese only showed a rebound of SWS after brief sleep deprivation in summer but not in winter. Also, both geese and starlings displayed strong seasonal variation in the overall amount of sleep. The starling in particular slept 5h per day less in summer than they did in winter. Moreover, both geese and starlings slept about 2h less during full moon nights than new moon nights. Another intriguing finding was the strong variation in REM sleep between the 3 species, which ranged from hardly any REM sleep in starlings to a much higher, mammalian-like amount of REM sleep in jackdaws. Such findings are difficult to reconcile with current theories in the function of REM sleep that are largely based on studies in mammals. Together, these findings in birds indicate that sleep is highly sensitive to environmental factors and suggest a great deal of flexibility in the regulation of sleep under natural conditions.