Adaptive Theory of Sleep
This theory suggests that patterns of sleep in different animals arose out of their relationship with the environment. For instance grazing animals that are prey to predators sleep in fairly short bursts. Adaptive theory says this is because they need to be alert, and longer or deeper periods of sleep would increase their vulnerability. Animals such as humans, gorillas and opossums sleep up to fifteen hours a day. Such animals have few predators. In the case of humans, the theory suggests that the long sleep pattern developed in the period when humans lived in caves, and were thus better protected.