A Songbird Dreams of Singing

A Note from the Author

Did you know humans are able to survive longer without food than they can without sleep? This rest time is so essential that people who get very little sleep each night are more likely to have health problems and live shorter lives. Infants need a great deal of sleep for normal brain development, and as we get older, we still need a lot of sleep to function properly. Given these facts, it is probably not surprising to learn that most of us will spend up to a third of our lives asleep.

Sleep isn’t just essential for humans, but for all animals. Although scientists are still unsure exactly why we need sleep, there are many theories. One is that sleep helps the brain repair different systems in the body, such as the immune, circulatory, and metabolic systems. Sleep may also be a time when the brain sorts through memories and decides which to keep and which to throw out. Another theory is that sleep allows our brain cells to repair themselves and recover from the day. It’s likely that sleep does not have a single purpose, but rather helps in many ways at once. Some research suggests that even animals without brains might sleep, like the jellyfish, who has only a simple set of nerve cells spread throughout her body. If this is true, perhaps there are even more basic reasons why sleep evolved, like the need to conserve energy. Whatever its function, sleep also seems to be necessary for very simple nervous systems.

In the animal kingdom, there is a huge range in the amount of sleep different animals need. The sperm whale may be able to get by on as little as an hour and a half per day, while the koala sleeps up to twenty-two hours a day. Because many animals do not have one uninterrupted sleep period like we do, the length of each “nap” can also vary. For example, worker fire ants sleep for a minute at a time, while giraffes may take five-minute naps, and lions may nap for five hours at a time. Although hibernation is a different state from sleep, bears may hibernate for up to seven months and snails may rest for up to three years! Still other animals—like dolphins and ducks—sleep with only half of their brain at a time, allowing them to rest while remaining on the lookout for predators.

When different animals sleep also varies. Diurnal animals—like sea otters and ducks—are active during the day and sleep at night. Nocturnal animals—like two-toed sloths and spring peepers—are active during the night and sleep during the day. Crepuscular animals, such as the ocelot, are most active during the hours of dawn and dusk, while cathemeral animals—like some species of lemurs—have active periods during both day and night.

Why do the sleep habits of animals vary so greatly? We can apply a few general rules that are usually true: Animals who sleep in a safe place tend to sleep longer. Animals who do not have to worry about predators can afford to slumber more, and animals who do not have to work as hard to find food also have more time to sleep.

In the pages that follow, you’ll learn that animals sleep in a variety of positions: upside down, standing on one leg, holding hands, while swimming, and even while flying! You will also learn that some animals are capable of dreaming. While there is still much to learn as scientists continue their research, I hope you find the sleep habits of animals as fascinating as the animals themselves.