Why do birds sleep with one eye open? Light exposure of the chick embryo as a determinant of monocular sleep

Together with some aquatic mammals, birds exhibit a unique behavioral and e lectrophysiological state called "unihemispheric sleep," in which one cereb ral hemisphere is awake and the other is sleeping [1], Slow-wave sleep in o ne hemisphere is associated with closure of the contralateral eye, while th e eye contralateral to the awake hemisphere is open; closure of both eyes, in contrast, is associated with bihemispheric slow-wave sleep or with REM s leep [1-3]. During the last few days of incubation, the chick's embryo is t urned in the egg so that it occludes its left eye, whereas light entering t hrough the shell can stimulate the right eye [4], Here we show that in the first two days after hatching, chicks coming from eggs incubated in the lig ht prevalently slept with their right eye open, whereas those coming from e ggs incubated in the dark prevalently slept with their left eye open. Thus, asymmetric light stimulation in the embryo can modulate the left-right dir ection of eye opening during post-hatching monocular sleep. (C) 2001 Elsevi er Science Ltd. All rights reserved.