Where do dolphins sleep at night?

Dolphins generally sleep at night, but only for a couple hours at a time; they are often active late at night, possibly matching this alert period to feed on fish or squid, which then rise from the depths. Bottlenose dolphins, based on electroencephalogram (EEG) readings, spend an average of 33.4 percent of their day asleep. read more

Dolphins sleep by resting half of their brain at a time. This is called unihemispheric sleep. The brain waves of captive dolphins that are sleeping show that one side of the dolphin's brain is "awake" while the other is in a deep sleep, called slow-wave sleep. read more

Within a 24-hour period, each half of the brain gets about 4 hours of slow-wave sleep, according to a 2008 article in the journal Neuroscience & Biobehavioral Reviews, which also notes there's scant evidence among dolphins for rapid-eye-movement, or REM, sleep (the stage in which dreams typically occur in humans). read more