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The internet is awash with anecdotes about high-powered CEOs touting their incredibly scant sleep schedules and early rise times. Pepsi’s former leader Indra Nooyi reportedly logs four hours of sleep a night. claim to need just six.

The thought of thriving on such little sleep—and freeing up so many more hours of your day— may sound tempting. But the reality is that the vast majority of us cannot function that way. Nor can you train yourself to need less shut-eye. A lot of people underestimate the amount of sleep their body ideally needs. And finding out how much you need may require some attention and experimentation.

How Much Sleep Do Humans Need?

Sleep scientists are in about adults needing at least seven hours of sleep per night for the best health outcomes. More specifically, adults aged 18 to 64 should get of sleep, and adults aged 65 and over should get seven to eight. published in Communications Biology found that older adults tended to make better and faster decisions, as well as have better working memory, when they got seven hours a night.

Despite all this evidence, there are plenty of people who claim to need or want fewer than seven hours of sleep. But often they confuse how much sleep they need for optimal functioning with how much sleep they just happen to be getting, says , an associate professor of psychology and neuroscience at Baylor University.

There are, of course, exceptions. “There are some individuals who can operate at a very high level on only a handful of hours of sleep,” says Scullin. Research shows there are a that cause people to be “short sleepers,” meaning they are highly functioning on just six hours of sleep a night.

Why You May Struggle to Find the Hours to Sleep

If your bedroom is too bright, loud, uncomfortable, warm, cold, or crowded, it’s going to be difficult to get quality sleep, says , an applied economist who studies sleep. Sure, you might be waking up early. But that might be because your body just can’t stay asleep, not because you’re well-rested.

Schofield studies the behavioral economics of sleep, focusing in particular on working communities in India. In her work, she found that when it comes to sleep and productivity, there are factors beyond the need for rest that can change people’s perception of how much sleep they need, especially when it comes to time.

“When you use time to sleep, you can’t use that time to do something else,” Schofield says. And so for some, their perceived “optimal” amount of sleep might be a shorter duration than what’s best for their health, simply because they can get more stuff done. It’s a cost-value trade-off that every population, regardless of nationality or socioeconomic status, has to consider, she says.

Calculating Your Optimal Amount of Sleep to Be Productive

To find out how much sleep your body requires for optimal focus and energy, Scullin says to make sure your sleep environment is comfortable, quiet, dark, and cool. Then, give yourself a lot of time to be in bed. Put your electronics away and don’t set an alarm. Simply lie down—and let yourself fall asleep and wake up at a natural time. “If you do that across one week, or maybe two weeks, then you’re eventually going to hone in on how much sleep your personal body actually needs,” Scullin says.

From there, it’s time to adjust your habits for the better. “The great news is, if you do increase your sleep, it’s pretty immediately rewarding,” he says. Sure, you may feel a little drowsy as your body adjusts to all those extra hours, “but as you get into a healthier sleep pattern, you’ll find your mood improves,” he says. “You’re probably even going to experience a happier outlook on life—and that’s really fulfilling.”