Just like dolphins, seals require only half of their brain to fall asleep.
The left side of their brain can sleep while the right side stays awake. Seals sleep this way while they’re in water, but they sleep like humans while on land. Acetylcholine, an important brain chemical, was at low levels on the sleeping side of the brain but at high levels on the waking side. This finding suggests that acetylcholine may drive brain alertness on the side that is awake.IANS