Sleeping is a way to conserve energy

I was thinking tonight about what the original purpose of sleep was and thought of hibernating bears. Hibernation in bears is a way for bears to survive the winter when there isn't any food(or very little).

Likewise, sleep is a way to conserve energy and body heat during the colder nighttimes until the daylight arrives again. So, sleep is the way mammals and other earth creatures including ourselves conserve energy and body heat so we can make it through the cold nights without the sun.