Many people think they can teach themselves to need less sleep, but they’re wrong, said Dr. Sigrid Veasey, a professor at the Center for Sleep and Circadian Neurobiology at the University of Pennsylvania’s Perelman School of Medicine.

We might feel that we’re getting by fine on less sleep, but we’re deluding ourselves, Dr. Veasey said, largely because lack of sleep skews our self-awareness. “The more you deprive yourself of sleep over long periods of time, the less accurate you are at judging your own sleep perception, ” she said.

Multiple studies have shown that people don’t functionally adapt to less sleep than their bodies need. There is a range of normal sleep times, with most healthy adults naturally needing seven to nine hours of sleep per night, according to the National Sleep Foundation. Those over 65 need about seven to eight hours, on average, while teenagers need eight to 10 hours, and school-age children nine to 11 hours. People’s performance continues to be poor while they are sleep deprived, Dr. Veasey said.

Extended vacations are the best times to assess how much sleep you truly need. Once you catch up on lost sleep and are not sleep deprived, the amount you end up sleeping is a good measure how much you need every night.

You can ask yourself the questions, “Do you feel that your brain is much sharper, your temper is better, you’re paying attention more effectively? If those answers are yes, than definitely get the sleep, ” said Dr. Veasey, who realized - to her chagrin - that she needs nine hours of sleep a night to function effectively.

Health issues like pain, sleep apnea or autoimmune disease can increase people's need for sleep, said Andrea Meredith, a neuroscientist at the University of Maryland School of Medicine.

A misalignment of the clock that governs our sleep-wake cycle can also drive up the need for sleep, Dr. Meredith said. The brain’s clock can get misaligned by being stimulated at the wrong time of day, she said, such as from caffeine in the afternoon or evening, digital screen use too close to bedtime, or even exercise at a time of day when the body wants to be winding down.