On average, we spend about a third of our lives sleeping — more than 9,500 days. That’s a lot of time, especially if you’re pressed for it. But sleeping is literally one of the most important things you can do for your body, and it’s responsible for so much of the body’s functioning that it’s hard to fathom how it has ended up being so dispensable for so many of us.

Sleep is necessary for the body to function properly — for tissue and muscles to rest, the immune system to recharge, the cardiovascular system to slow down, and metabolism to stay in rhythm. It’s crucial for your mind to work the way it’s supposed to too: It helps the brain clear away clutter and consolidate important information learned during the day. “Sleep is one of our basic functions, and it is important because being awake is very energy-consuming, and it's very stressful,” says Sara Mednick, Ph.D., a researcher at the Mednick Sleep and Cognition Lab at the University of California, Irvine.

Various pieces of research have linked sleep deprivation to poor working memory. Sleep quality and length are also linked to emotional intelligence, according to a 2022 study. And insomnia is correlated with higher likelihood of developing depression. “Your brain at night is basically flushing the toilet and letting all the stuff go,” Mednick says.

Yet more than 35% of adults in the U.S. don’t get enough sleep, according to the Centers for Disease Control and Prevention (CDC). Of course, sometimes that’s inevitable, like when you have a baby who isn’t ready to sleep through the night. So if you’re waking up every few hours to soothe or feed or cradle, how little sleep can you get and still be a functioning adult? Is six hours of sleep enough? What about four, or God forbid, two hours of sleep?

Your Body On Eight Hours Of Sleep

“By the definition, seven to eight hours, on average, should be the amount of sleep one is looking for. That’s good quality sleep — sleeping through the night with no disturbances or minimal disturbances,” says Peter Polos, M.D., Ph.D., a sleep medicine specialist based in New Jersey.

When you get less sleep than that day after day, the cumulative effects add up proportionally, says Polos. That’s called going into “sleep debt.” Polos adds, “But there are no frank plateaus where at six, this happens, at four, that happens. It's a slide. As the sleep deprivation continues, the consequences become worse and build up.”

Eight hours isn’t an arbitrary number. Already in 1938, Nathaniel Kleitman — credited as the discoverer of the REM sleep phase and father of sleep science — spent 32 days cooped up in a dark, gloomy cave with a student of his to study how much they would naturally sleep without any sunlight. What happened is that their bodies resorted to eight to eight-and-a half hours of sleep per night.

In 2018, scholars at the University of Western Ontario backed up this finding with one of the biggest sleep studies ever performed, comparing how 10,000 participants fared after getting varying levels of sleep. The researchers looked at, for example, performance on cognitive exercises after participants slept less than eight hours, eight hours, or more than eight hours. They found that those sleeping in that middle range outperformed everybody else. Envision it like a bell curve where eight hours gives rise to maximum performance, and getting any other amount of sleep puts you at a disadvantage.

Your Body On Six Hours Of Sleep

Quick thinking and reflexes are among the first to go when you drop from eight hours of sleep, says Polos. In his sleep training program, he runs an experiment in which participants must click a number that pops up on the screen as soon as they see it. “From this, we know that with increasingly less and less sleep, that reflex time gets longer and longer,” he says. “You begin to feel very lethargic. Everything you need to do becomes more and more effort.”

Much of what’s known about sleeping less than the recommended time comes from a 2003 study led by David Dinges, Ph.D., chief of the division of Sleep and Chronobiology at the University of Pennsylvania School of Medicine. His team ran tests on (willing) participants, forcing them to undergo systematic sleep deprivation. Cohorts spent two weeks either getting eight, six, or four hours of sleep — and one group had to go three days without any sleep at all.

Every day, people sleeping six hours or less saw their performance plummet on cognitive tasks testing their alertness and ability to reason, communicate, and remember. Their abilities declined steadily with almost every passing day. By the sixth day of the experiment, a quarter of the participants were falling asleep while performing their tasks. After two weeks, people getting six hours of sleep every night were performing just as poorly on cognitive tests as those who had been deprived of sleep for a whole night straight.

Out in the real world, getting six hours of sleep on a regular basis can put you in physical danger, as it increases your risk of getting into a car crash by 33% compared to sleeping 7 or 8 hours, according to a 2018 study.

Getting six hours of sleep per night also takes a toll on your metabolism. “Studies show that people with five days of six hours of sleep start to have blood glucose tolerance levels that look like they're getting to prediabetic ranges, because they have increased levels of the stress hormone cortisol,” says Mednick. That’s because sleep keeps your metabolism in check. Poor sleep is also linked with a higher risk of obesity and weight gain, according to this 2018 review and this 2020 review of research.

Getting even just one hour less than the recommended eight hours also makes you more selfish. In a study published in 2022, researchers showed there was about a 10% decrease in online charitable donations across the whole U.S. population in the weeks after Daylight Savings — when people get one less hour of sleep for one night only. The same study also found that participants who slept less across several nights were less likely to choose to help others during a test of their altruism.

Your Body On Four Hours Of Sleep

Participants who slept for only four hours in Dinges’s experiment fared significantly worse than their six-hour counterparts. After just three days, it was like they’d gone a whole night without sleep, according to their cognitive test results. By day 10, they were performing as if they had been up for 48 hours straight.

“Clearly the less sleep you have, the shorter duration you have to function reasonably well,” says Polos. “If you're down to four hours, you might be able to get by for a couple of days.”

What’s more, you’re probably not likely to realize the cognitive differences. Participants in the Dinges study self-reported having adjusted to the new levels of sleep, even as they were bombing their cognitive exams. They felt tired, sure, but they thought the sleepiness wasn’t affecting their cognitive ability.

The longer you limit yourself to four hours of zzz’s, the worse the outcome. The previously mentioned 10,000-participant sleep study, for example, found that those who regularly got just four hours of sleep per night had aged their brains by about eight to nine years more than their counterparts who were getting eight hours of sleep.

There are also consequences to the body. “The damage from sleep loss is probably not irreversible, so you're not suddenly immunosuppressed and immunocompromised because your immune system's been taxed out,” says Polos. “But you may get a cold more frequently.”

Your Body On Two Hours Of Sleep

Functioning on two hours of sleep is not feasible, especially not on a regular basis.

For most people, sleeping this little or having very disturbed sleep can lead to very scary consequences. Some of the best evidence of this comes from hospitals. “Sleeping in hospitals as patients is almost impossible and actually leads to worse health for the patients,” says Mednick. “They totally mess with people's circadian rhythms, and in the ICU in particular because they're constantly checking on people and they have this constant light. So it can lead to something called ICU psychosis, where the patients get super paranoid and they have hallucinations.”

Still, any amount of sleep is better than nothing. Because of the way sleep phases work, you want to get at least 90 minutes of sleep to go through a whole REM cycle. And research has shown that 90 to 110 minutes of sleep is significantly better than 60 minutes of sleep in terms of feeling rested and the sleep having a restorative effect on your body.

Your Body On No Sleep

Sometimes pulling an all-nighter is the only option. It won’t screw you up forever, but it does come with consequences.

A study published in 2014 showed that being sleep-deprived for one night can increase emotional response and negative feelings by about 60%. And CDC research shows that being awake for 18 hours causes the same mental confusion and impairment as having a few drinks — having a blood alcohol content of 0.05%. Going a full 24 hours without sleep is equivalent to having 0.10% blood alcohol content.

Still, there is no scientific record of somebody dying from sleep deprivation. “There's a myth going around that if you don't sleep enough, you'll just die,” says Polos. “If you’re sleep-deprived for five days, you will be in pretty bad physical and mental shape, but you won't be dead.”

The longest recorded waking time is about 264 hours, or 11 days, the Guinness world record set by teenager Randy Gardner for an experiment in 1953. But don’t be Randy. Please, just hit the hay.

Making Up For Lost Sleep

Luckily, it is possible to catch up on your sleep debt — to a point. “If you have one or two nights where you're behind your normal amount of sleep and you can sleep in on a Saturday or Sunday, as an example, you can sort of rebalance the equation,” says Polos. The problem is that when you only sleep six hours Monday through Friday, you're down 10 hours now, and you're not going to be able to make up 10 hours on a weekend.

When Dinges’s study participants were allowed three days to recover and make up for their missed sleep, they still didn’t do as well on their cognitive tests as when they had started the trial.

The solution? Enter napping, the ultimate tool to combat sleep deprivation.

Getting enough sleep doesn’t necessarily mean you have to get all eight hours at once. “We sleep in multiple phases,” Mednick says. That can include one phase before waking up in the middle of the night, one phase after, and a nap the next day. “All of these are sleep, and they're all the same type of sleep. They're all really good sleep,” says Mednick.

And for new parents struggling to get a full night’s rest, she adds, “The kind of sleep you had pre-baby is not what you're gonna have post-baby, but that doesn't necessarily mean you don't get to be well-rested.”

This article was originally published on Sep. 22, 2022