We spend roughly a third of our lives asleep. Snoozing is vital to our well-being — no human can go without it for more than a handful of days. And yet it is perhaps the least understood of all our activities.
Theories abound, but much of our need for sleep remains a mystery. Sleep certainly allows for a lot of body "maintenance work," from production of chemicals that get used during waking hours to the self-organization of neurons in the developing brain. REM sleep, with its high neuronal activity, occurs more during periods of brain growth.
Several theories point to sleep as a state vital to memory and learning abilities. Sleep may help ingrain episodic memories into long-term storage, and it also may simply give our waking activities a much-needed break.