In an ideal world, we would all wake up in the morning feeling refreshed and ready to face the day. But if you’re anything like me, you can barely pry your eyes open when your alarm goes off and your bed seems to exert a strong magnetic pull on your body.
Enter the 90-minute sleep cycle hack, which promises to help you rise feeling energized. The idea is to time your sleep and wake times in such a way that you sleep in 90-minute increments. This means counting backward from your desired wake-up time in 90-minute blocks to find your ideal bedtime. (There are several calculators on the internet that will do this for you.) For example, to wake up at 7:00 a.m., you could go to bed at 10:00 p.m. for six 90-minute cycles (9 hours of sleep) or 11:30 p.m. for five cycles (7.5 hours of sleep).
The trend became so popular on social media that almost one in 10 American adults have tried it, according to a 2024 survey by the American Academy of Sleep Medicine. But is there any scientific evidence behind this ‘sleep hack?’
Where the 90-minute rule comes from
The concept dates back to the 1950s, when researchers discovered that sleep happens in cycles (which they called ultradian cycles), says Dr. Fouzia Siddiqui, the medical director of the Sentara Rockingham Memorial Hospital (RMH) Sleep Center. Your brain and body journey from light stage 1 and 2 sleep, to deep stage 3 sleep, followed by a period of rapid eye movement (REM) sleep with dreaming, before the cycle begins again.
Light sleep serves as a transition from being awake to being in deep sleep. Deep sleep is when your body repairs and restores itself, and dream-filled REM sleep allows your brain to process emotions and consolidate memories.
A single sleep cycle typically lasts around 90 minutes, says Dr. Andrea Matsumura, a board-certified sleep medicine physician and founder of the Sleep Goddess Method. “Most people complete four to six cycles per night.”
The logic behind the 90-minute sleep cycle hack is simple: Waking up at the end of a completed cycle, when you are in a lighter stage of sleep, makes you feel more refreshed than if you are jolted out of deep sleep.
Why the hack doesn’t always work
The problem is that real sleep is messier than textbook descriptions. Matsumara explains that while sleep cycles average around 90 minutes, their length varies from person to person.
Even in the same person, a sleep cycle doesn’t always last the same amount of time. “Factors like stress, alcohol, illness, or even what time you went to bed all impact your sleep cycle so this method isn’t really a guarantee that you’ll wake up less groggy,” she says.
Jade Wu, a board-certified sleep psychologist, agrees that the 90-minute sleep cycle hack is not scientifically sound. “Sleep cycle length varies depending on how much sleep you need that night, hormones, your sleep environment, what you ate, what you did, where you are, what season it is, what time you went to bed, how much you’ve slept recently, and more,” she says. “There’s no way to predict or regulate cycles to be exactly 90 minutes.”
As the night progresses, sleep cycles tend to get longer, with the first sleep cycle ranging from 70 to 100 minutes, and later cycles lasting between 90 and 120 minutes.
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In theory, the 90-minute sleep cycle hack promises to help you avoid waking up from deep stage 3 sleep, which is more difficult and leaves you feeling groggy. But Wu explains that the hack doesn’t make sense because sleep cycles change over the course of the night. Early in the night you spend more time in deep sleep, while the later hours are increasingly dominated by REM sleep. “In fact, in the second half of the night, healthy sleepers usually don’t get deep sleep at all—so people shouldn’t be waking up out of deep sleep in the morning anyway,” she says.
What really helps you wake refreshed
“Instead of relying on such a hack, it’s better to prioritize a consistent sleep schedule, so your body naturally wakes up closer to the end of a cycle,” says Matsumura. This means going to bed and waking up at the same time—even on weekends.
Research shows that adults between 18 to 65 years need between 7 to 9 hours of sleep, says Siddiqui. (That is, unless you are one of the lucky few who thrive on six hours of sleep or less.)
Both Siddiqui and Matsumura advise avoiding screens close to bedtime and keeping your room quiet and comfortable to help you get a better night’s sleep. Also limit sleep disruptors like alcohol and caffeine in the evening.
Wu says that it’s normal to feel groggy for 20 to 30 minutes after waking, even with healthy sleep. Natural sunlight in the morning can help to wipe away your grogginess, says Matsumura. It resets your internal clock and boosts alertness by signaling to your brain to reduce melatonin (the sleep hormone) and increase cortisol (for alertness).
If daytime fatigue lingers, Wu recommends screening for sleep disorders such as apnea, reviewing medications with your doctor, since these can significantly affect sleep and daytime alertness.”
Bottom line
The 90-minute sleep cycle hack may sound scientific, but it doesn’t hold up under scrutiny. Experts agree that maintaining good sleep hygiene is far more effective than chasing calculator-approved bedtimes.
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.