In a below-ground classroom at the Lincoln Park Zoo—somewhere between the monkey house and the lion enclosure—Chicago pediatrician and sleep expert David Gozal was explaining how migratory geese sleep in formation, each bird dozing with half its brain, while the other half keeps watch on the flock. “So they can fly for hours and hours and hours and still get their sleep,” Gozal said, a tinge of astonishment in his voice. “And they rotate so everybody gets the same amount of sleep. It’s fantastic. Extraordinary.”

It was a little after 10 a.m. on a Saturday in mid March. As the rest of the zoo awakened to the day’s first visitors, Gozal’s drowsy audience sipped coffee while he and zoo biologist Rachel Santymire described the sleep habits of humans and wild animals, how non-REM gives way to REM and then back again, how all of it eventually gives way to deeper delta sleep, how sleep paralyzes every muscle but the diaphragm, allowing us to breathe while preventing us from acting out our dreams. Humans’ body temperature drops at night by as much as three degrees centigrade, Gozal said. “Your body becomes less like a mammal and more like a fish.” The phenomenon is, in fact, he said, an evolutionary holdover from the days when animals first climbed out of the ocean and walked on land.

Santymire explained that meat eaters’ high-density diets allow them more time for sleep than herbivores, which must graze longer to get enough calories. “Also, any way you can reduce your caloric needs by sleeping next to something warm, it’ll increase your length of sleep,” she added, putting up a slide one of her grad students took in Tanzania of a featherless chicken huddled so close to a puppy and a dog that their bodies were almost indistinguishable.

Santymire studies endangered black rhinos in South Africa and has lately been wondering whether sleep patterns might offer an indication of environmental pressures. So last year she launched a study to find out. “This is the first time that sleep behavior has been characterized in black rhinos, let alone wild black rhinos,” she said. She and her team have found that the animals sleep about 90 minutes at night and probably more during the day—she showed photos of rhinos lying motionless under trees or hidden in the grass (“look for the big boulders,” she said), captured by infrared cameras.

Bats can sleep for as many as 23 hours a day, Gozal said. “Sometimes they only have one hour to zoom through as many insects as possible.” Insects sleep too. “Every animal we have been able to identify sleeps,” he said. “Worms sleep too, very nicely. We call that ‘lethargus.’”

The sleep that interests Gozal most, though, is people’s. And specifically, children’s. On average, he said, adults need eight hours and 23 minutes of sleep. “Raise your hands if you sleep eight hours and 23 minutes,” he instructed listeners. No hands went up. Kids, he said, need more and deeper sleep, as many as ten hours per night. Too often, they don’t get it. Sleep deprivation raises children’s risk of obesity, diabetes, and cardiovascular disease, he's found. Sleep is important, he said, “as a way for many of the early experiences that are very overwhelming to be processed.” Without enough sleep, “especially early in life, the development of what you will become—metabolically, mood, intelligence, everything—will be affected.”

During the Q&A, Gozal explained why kids act up instead of settling down when they’re tired. “Children usually resist going to sleep,” he said. “You will notice that you don’t fight with an adult to go to sleep.” The reason? “Their brains are not yet developed, so the connectivity is different. The areas that control impulsivity, the frontal part of the brain, will reduce in activity, and so the child becomes disinhibited. They become hyperactive, hyperaggressive, hyperemotional. Because the control has been taken away.” When kids reach 12 to 15 years old, “these connections are different, and the control is much tighter.”

By the time the lecture ended and listeners emerged into the daylight, it was approaching noon. The whole place was thronged by families wheeling strollers, trailing children. At the zoo’s northern edge, a brown bear was sleeping next to a rock, his back to visitors. “Look! Let’s wake him up!” a little boy shouted to his mother as he ran toward the fence. “No,” she said. “Let’s let him rest.” For a long moment they stood silently watching the bear, its rib cage rising and falling, rising and falling, before they turned and walked on.