Sleep is a time suck. If you multiplied the average recommended number of hours we should sleep in a day—eight for a typical adult—by the number of days in an average lifespan (78.8 years in the United States), that would amount to about 9,587.3 days. That's one third of your life spent unconscious. From an evolutionary standpoint, sleep is quite literally a waste of your time, yet it's fought its way through countless years of adaptation in nearly every living animal on Earth. So it must be important, right?

In fact, researchers have found that sleep plays a vital role in the functioning of nearly every organ system in the body. At the same time, medical conditions, a busy schedule, and even the simple unavoidable act of agingconstantly challenge the number of hours we allow ourselves. But that begs the question: how much sleep do we actually need? And can we train ourselves to need less?

First, let's talk about that eight hour figure that gets tossed around. It's far from some arbitrary number. It's truly the number of hours we naturally crave, and there are two pretty strong pieces of evidence for it. In a series of experiments, researchers took study participants into a laboratory with no sunlight or other visual cues and, at night, gave them a non-negotiable, nine-hour-long opportunity to sleep. They did this each night for a number of weeks, and the results were always the same: even when provided with more time, humans will typically spend an average of eight hours catching up on their Zzz.