The Secret World of Animal Sleep

In the animal kingdom, the higher up the food chain you are, the more sleep you get. That's why prey species like giraffes can only afford mere minutes of sleep at a time, while lions revel in over 20 hours of luxurious slumber a day. Take an eye-opening look at sleep as a survival strategy.

We know marine mammals like dolphins need air to breathe, but what happens when they rest? The solution is a fascinating form of half-sleep that allows them to emerge for air without waking up. Dive into the weird world of underwater slumber.

When birds need sleep, they do it on the fly--quite literally. They're able to enter a special kind of half-sleep state that lets them rest midair, which is especially important if you're a migratory bird, like the Alpine swift, clocking thousands of miles at a time. Glide into the astounding world of avian sleep.

Owls and foxes are born with undeveloped brains. And to grow, they need a special kind of deep sleep known as REM, or rapid eye movement--a state marked by brain regeneration, memory development, and a lot of movement. Take an intriguing look into the role of sleep in helping different species develop.

Arctic ground squirrels survive harsh Canadian winters through skilled hibernation. Entering a state of controlled hypothermia, their body temperature drops to 27 degrees F--the lowest-ever recorded in a mammal. This suspended animation isn't sleep exactly, but it's an equally vital part of their unique survival plan.

All animals sleep--but few do so in exactly the same way. Horses snooze standing up, and snakes sleep with their eyes wide open. As for some species of jellyfish, they sleep despite lacking a brain! Uncover some of the more unusual features of animal slumber, many of which remain a mystery to us.