You may have heard that dolphins never sleep, but in fact dolphins are known for unihemispheric sleep, in which half of the brain sleeps while the other half stays active. Adult orcas normally snooze 5 to 8 hours a night, closing both eyes and floating motionlessly. Newborn killer whales and dolphins sleep in an unusual way. For the first 30 days after birth, neither the calf nor its mother shuts its eyes, and over the next months both gradually rest more until they reach normal adult levels.
Northern fur seals show half-brain sleep in a clear way. During one kind of sleep, called slow-wave sleep, their brain waves are strongly lopsided, with one half of the brain more asleep than the other. The brain chemical acetylcholine is released unevenly, much more on the awake side. A northern fur seal can also briefly open one eye while a recording shows one half of its brain asleep, and birds that sleep with half a brain close only one eye.
Great frigatebirds put half-brain sleep to a hard test. Scientists tracked them on flights lasting up to ten days and 3000 kilometers, with a small recorder on the head that registered the activity of both brain halves. The birds could sleep in flight, using one half of the brain at a time or both halves together. But they slept for only about 0.69 hours a day. Other birds may sleep in the air, too. Alpine swifts stayed in the air for about 200 days, from their trip to Africa through the winter, and common swifts spent about ten months of the year almost constantly in the air. If these birds sleep during such trips, they must do it while flying.
Elephant seals sleep while diving. Scientists recorded brain activity in wild seals with a watertight cap. They found that at sea the seals averaged just 2 hours of sleep a day, compared with 10 to 11 hours on land. On deep dives of about 30 minutes, they nap for about 10 minutes at a time while holding their breath, and diving elephant seals enter slow-wave sleep while gliding down, then REM sleep, when sleep paralysis makes them turn upside down and drift in a sleep spiral.
Elephants sleep very little, only about 2 to 3 hours a day. Two wild African elephant matriarchs fitted with trackers slept about 2 hours a day on average, mostly between 2 a.m. and 6 a.m., and one matriarch went without sleep for up to 46 hours and walked about 30 km in 10 hours. Wild matriarchs lay down to sleep only every third or fourth day.
Sleep, torpor and hibernation are three different low-energy states that share slowed heart rate and lower body temperature. A hibernating ground squirrel can drop its metabolic rate by up to 90 percent. In an Alaska study hibernating black bears functioned at only 25 percent of their active metabolic rate even though their body temperature did not drop much. Hibernators also wake on a schedule: depending on the squirrel, torpor is interrupted every 14 to 21 days, when the body warms to about 37 degrees Celsius and the squirrel stretches and then goes back to sleep. The dwarf lemur does not keep a steady body temperature while hibernating: it can swing by as much as 36 degrees Fahrenheit, depending on how well the tree hole is insulated.
Bats sleep hanging upside down, about 20 hours a day, and tendons in their feet let them hang while perfectly relaxed, without working a muscle to hold on. The roost position has a payoff, too. Hanging upside down, a bat that is relaxed can let go and drop into flight, whereas bats that sat upright would have to contract a muscle, and so spend energy, to let go and begin flying.
How do you prove that a whale is asleep? Scientists call a rest sleep when the animal is quiet, reacts less to things around it, and sleeps more after being kept awake. Sperm whales float upright, almost motionless, with their heads at or just below the surface. Researchers brought a boat near floating whales, and the whales did not react, even though the boat was visible to both eyes, until it accidentally touched one. The whales were called asleep by a standard test, not by brain waves, which looks for reduced responsiveness that reverses with enough stimulation to cause waking.
Scientists also tagged 59 sperm whales around the world with suction cups and gathered 562 hours of data. The whales spent about 7.1 percent of the tag time in resting dives, most often between 6 pm and midnight. That is not their total sleep, since it counts only the time the tags were on. So scientists can call a rest sleep by how an animal reacts, not only by its brain waves.
Words to know
- acetylcholine: a brain chemical
- unihemispheric: one half-brain
- paralysis: unable to move
- torpor: deep rest
- matriarchs: lead females
