If you’re the kind of person who needs at least 8 hours of sleep before you can operate the toaster, the following news might make you jealous. Giraffes, no matter how far they gallop in a day, need only 20 minutes of deep sleep a night.
Reptiles and amphibians don’t sleep deeply at all, but at least they rest.
Hyperactive shrews are so consumed with finding their next earthworm meal that they don’t even slow down!
Sleep or no sleep, many animals can still run, leap, lurk, flap, and lunge with precision, which is more than you can say for humans. When we lose too much sleep, we actually suffer from mental anguish. Are we victims of some evolutionary quirk when it comes to downtime? Not really. On the scale of slumbering, we are not nearly the most lethargic. Koalas sleep for 18 of the 24 hours, squirrels for 14, and lions a lazy two-thirds of their lives compared with our one-third.
The extreme disparity in sleep requirements among animals has experts battling for a hypothesis. At present, there are two main camps of thought. One says that sleep recharges our batteries, but not necessarily our physical ones. If we wanted to refresh only our bodies, we could do that by lying down and relaxing all our muscles, say researchers. They claim the real reason for deep sleep (complete with dreaming) is to recharge our mental batteries, to sort through the sensations of the day and file them into place. As our memory banks tidy up the information, hard-to-categorize pieces come back as awkward dream sequences. Five of these dream sessions occur on a typical night, leaving us refreshed and able to find our car keys in the morning.
This recuperative theory, though it seems to work well for humans, bogs down when we begin looking at other animals. If recuperation is a must, wouldn’t the high-metabolism, busy animals like shrews need it the most? And wouldn’t slow-moving, lethargic animals like sloths have less to recuperate
from? Then why is it that slothful sloths sleep 20 hours a day but harried shrews don’t sleep at all? If recuperation is for restoring mental powers, wouldn’t humans need more sleep than, say, armadillos?
Perhaps, claims the other camp of sleep theorists, animals are doing something else when they sleep. Maybe sleep is not the great recharger, but merely the great immobilizer. Maybe sleep knocks animals out during the times of day when their adaptations fail them: when it is too hot, too cold, too dark, or too bright to function safely. Asleep during these off-times, they may be less visible to predators than they would be if they were awake and fretting over every disturbance.
Of course, how long you sleep depends on whether you can afford to be immobile. If you must constantly eat to stay alive, a long sleep would be suicidal. Or if you are intensely vulnerable to predators both day and night, the way giraffes and other animals on the plains are, your sleep must also be brief.
Some sleep habits may instead hark back to days when prehistoric predators made caution a survival imperative.
Today, the luxury of lethargy is often reserved for those at the top of the food chain, as well as animals that are large enough or heavily armored enough not to worry. As always, there are exceptions. Although elephants haven’t worried about natural enemies for eons, they snooze for only 2 hours a night. For them, it’s not the pressure of a predator that keeps them awake, but the pressure of their own enormous weight bearing down on their organs. To soften the blow, elephants often bunch up grasses beneath their head and flank before they turn in.
Other animals also prepare for a good night’s (or day’s) sleep, and it’s one of the behaviors you can look for at the zoo. Gorillas use vegetation or straw provided by zookeepers to create a soft, springy bed. Peacocks ascend by the dozens to communal roosts in tree branches, a habit that may give them more than just a good night’s sleep. Researchers think group roosts may double as information centers, where birds stand to learn about the good feeding spots their comrades found that day.
Flamingos also sleep in a group, but they stand up, tucking their beak behind one wing and keeping one eye open for predators. Dolphins are one-eye-open
sleepers as well. By switching eyes every now and then, they can shut down one hemisphere of the brain at a time, resting both halves without ever being completely vulnerable.
One of the problems zoos face is that not all animals sleep at night the way most humans do. There are those that work during the day (diurnal), those that roam at night (nocturnal), and those that are active only during dawn and dusk (crepuscular). In the wild, this staggered sleep-shift allows different species to fully utilize a habitat without getting in one another’s way. Zoo designers have come up with a way to keep night workers awake even when it’s high noon. By keeping their exhibits dark during the day and bright at night, they induce nocturnal animals to switch their biological clocks and sleep during the same hours that we do. The red lighting (which animals see as black) in “nocturnal world” exhibits allows us to see them operating in what they think is the dead of night.
If you do happen to catch animals sleeping, however, don’t be disappointed.
Remember that sleeping is a very legitimate and necessary activity, one that we ourselves spend 20 to 30 years doing. Besides, it’s fun to try to identify the sleep cycles of various animals. Higher mammals and birds experience periods of deep sleep alternating with periods of light sleep. You can look for evidence of rapid eye movement (REM) under the eyelids along with muscle twitches and occasional changes in posture.
SLEEP REQUIREMENTS. To make the most of their nutrient-poor diet, three-toed sloths are careful not to burn too much energy. When they aren’t sleeping (which they usually are), they operate in extra slow
motion.