Figure 2: Unihemispheric and bihemispheric sleep in flight.

(a) Recording of head acceleration in three dimensions (sway, surge and heave) and electroencephalogram (EEG) activity from the left (L) and right (R) hemispheres showing the transition from wakefulness to SWS following the cessation of flapping (red bars). Brief episodes of dropping (green bars) occur after this episode of sleep. Expanded views (bottom) show wakefulness characterized by low-amplitude, high-frequency EEG activity in both hemispheres, infrequently punctuated by isolated high-amplitude, slow waves (*), and SWS characterized by continuous high-amplitude, slow waves, in this case, primarily in the left hemisphere. The red arrow (top) marks an episode of apparent REM sleep (expanded in Supplementary Fig. 13). (b) Recording from the same bird showing an episode of bihemispheric SWS (BSWS) and unihemispheric SWS (USWS), including expanded views of both states. The mean duration of episodes of sleep was shorter than these long episodes used to demonstrate USWS and BSWS in flight. These recordings are from frigatebird 13 (Supplementary Fig. 7).