A great frigatebird can stay out over the ocean for weeks without landing, and for a long time nobody knew how it slept. In 2016 a team led by Niels Rattenborg put small recorders on the birds’ heads and found out: in short naps, a few seconds at a time, mostly while circling up in rising air, sometimes with half the brain asleep and the open eye turned the way it was going. Added up, it came to about forty minutes a day. The same birds, back on land, sleep for something like twelve hours. So the answer was not that the frigatebird has learned to do without sleep. It has learned to put it off, and it pays when it gets home.