Dolphins sleep uni-hemispherically, with half their brains awake. Newborn dolphins keep one eye open for a month. They flip sides to rest. This avoids sleep-attack sharks. Sleeping dolphins.
Orca infants spend their first few months without sleep. Young orcas swim even as the adults slumber. Calves need to move continually to keep warm. Once they have blubber, they can rest and sleep.
Fruit flies sleep differently. Some flies sleep 10 hours a day, while others sleep only 4 minutes. In an experiment, sleep-deprived fruit flies lived as long as typical sleepers.
Bullfrogs stay alert and react to stimuli even when sleeping. Some consider brumation binge napping after no sleep. They "relax" but never sleep the other months.
Fish sleep differently when caring for young or after birth, in addition to migrating. Tilapia are awake and active for 22 weeks after birth. Adults enter a sleep-like state, while children don't.
Say sleep means your brain is inactive. Animals like upside-down jellyfish without brains can't sleep. They feature a network of sensors and tentacles instead of a brain to feed on microscopic water animals like zooplankton.
Butterflies that can't sleep. Torpor is characterised by a slow heart rate and a low core body temperature. They prop their open eyes on leaves or twigs.
Walruses may look sluggish, but they can stay awake for 84 hours. They'll swim and hunt for food. After long periods of non-sleep, the walrus will take a fascinatingly long nap.