For an astonishing 11 days and 25 minutes, 17-year-old Randy Gardner didn’t sleep for a high school science fair project in California in 1963, setting the world record for the longest someone has stayed awake(opens in new tab). Other people have reportedly broken this record — Robert McDonald went 18 days and nearly 22 hours without sleeping in 1986 — but none were monitored as closely or by a doctor as Gardner was.

Guinness World Records doesn’t cover this feat anymore; in 1997 they stopped accepting new submissions due to “inherent dangers associated with sleep deprivation.”

But what are these dangers? What happens to people who experience prolonged sleep deprivation?

Sleep is necessary for executive, emotional and bodily functions, and insufficient sleep may raise the risk of several health conditions, including diabetes, heart disease, obesity and depression, according to the Centers of Disease Control and Prevention(opens in new tab). Experts say humans require a consistent six to eight hours of sleep at the same interval every 24 hours. But it’s not uncommon for people, especially students, to pull an all-nighter and stay awake for 24 hours.

At this stage of sleep deprivation, it can be challenging to distinguish between sleep and wakefulness, said Dr. Oren Cohen(opens in new tab), a sleep medicine fellow at Mount Sinai Hospital in New York City. As someone starts to push 24 hours without sleep, their brain activity already shows signals that they are on the sleep-wake border, even though they appear to be awake, Cohen said.

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