man napping sleeping park bench snow winter RTX187Z9 Reuters

We all do it. We all like it. We all need more of it.

We've known for decades that sleep is important. Without it, we'd all be a cranky, sick mess.

But one question has plagued researchers for more than a century: Why do we sleep, anyways?

"You'd think I'd have an answer for that, right?" Patrick Fuller, associate professor of neurology at Harvard Medical School told Tech Insider. "I've been doing this for a while and I can't answer that question."

Scientists have been formally studying sleep since the early 1900s, but Fuller is right — we still have absolutely no idea why we need to sleep.

Theories abound. Some say that it allows us to be rested and active during the day, when we can forage for food (you can see berries much better in the sunlight than in the darkness).

Others say that it's primarily restorative, allowing the body to grow muscle, repair tissues, synthesize proteins, and release growth hormones.

Recent research suggests that sleep may also help the brain flush out gunk and brain-cell-destroying toxins that build up during the day.

But perhaps the most compelling theory is that sleep helps shape and organize the brain, which eventually helps us learn and remember things. We still don't fully understand this phenomenon, called brain plasticity, but we know that it's important — specifically for a developing young brain.

For instance, infants snooze for 13 to 14 hours per day. Much of that time is spent in deep REM sleep — the stage when the most dreaming occurs. This is crucial for the trillions of new connections formed within the first year of life.

A man attempts to sleep on his suitcase at John F. Kennedy International Airport in New York January 22, 2014. REUTERS/Andrew Kelly A man attempts to sleep on his suitcase at John F. Kennedy International Airport in New York Thomson Reuters

And still, while we haven't proven the "why" of sleep, we do know one thing: it's necessary for survival.

Just peruse any rodent study that completely deprives them of sleep.

"What you find is that they do die," Fuller said. "They start to develop ulcers on their organs, they go into thermodiscontrol [can't regulate body temperature] — they end up dying."

We can't exactly replicate this experiment in humans; that would be tantamount to torture. But there are cases of people getting severely ill or dying after long periods of sleep deprivation (it's hard to say that sleep alone was the cause of these deaths, but we do know for sure that sleep deprivation does a number on the body).

After depriving yourself of sleep, Fuller said, first you start hallucinating; then your ability to do, really, anything effectively deteriorates.

"You're almost like an automaton, walking around doing stuff," Fuller said. "It's almost like your body is moving but your brain is not communicating that information."

In studies where humans only get four hours of sleep per night for two to three weeks straight, the hit to health is staggering. Blood tests show cardiovascular stress, bad metabolic changes, lowered immune function, and inflammatory markers.

"Basically you look at the profile of these people and you say, 'hmm,' this person looks really sick to me," Fuller said. "That's what we see when we restrict sleep in humans."

Technological advances have allowed us to map out brain circuits and synaptic connections involved in the processes of falling asleep and waking up, Fuller says. This clues us in to more of the "how" we sleep rather than the "why" we sleep. But even then, there's still a ton more we don't understand.

And with an estimated 50 to 70 million adults in the US suffering from a sleep or wakefulness disorder, according to the Centers for Disease Control and Prevention, understanding the "why" can take us one step closer to actually treating some of these issues.

"If you know the why," Fuller said. "then you can address issues of sleep deficiency a lot better."