Most experts recommend getting seven to nine hours of sleep a night — so why do we sometimes get a full night’s rest and still wake up feeling tired?
It may have something to do with the point in the sleep cycle that we wake up. And according to Dr. Charles Puza, a New York City dermatologist and self-professed biohacker, we actually have some level of control over that.
Puza says that a simple math formula can reveal your “ideal bedtime” so you wake up feeling your best — and it’s not just about hitting the sack earlier.
“Does your nap feel more restful than a full night’s sleep? Ever wake up from 8+ hours of sleep and still feel groggy?” Puza asked his 811,000 Instagram followers.
“It’s because you’re going to bed and waking up at the wrong time. You should be timing your sleep to align with natural sleep cycles of around 90 minutes.”
The sleep style goes through four stages. According to the Sleep Foundation, stage 1 can last for 1–7 minutes, stage 2 is 10–25 minutes, and stage 3 — also known as delta sleep or deep sleep — is 10–40 minutes.
REM sleep — when we dream — can last anywhere from 10 to 60 minutes.
Though that obviously means the length of an entire sleep cycle can vary, the average duration is an hour an a half, which Puza has based his formula on.
With that in mind, six full sleep cycles of 90 minutes each equals 7 hours and 30 minutes. Add another 15 minutes to give yourself time to fall asleep, and you arrive at 7 hours and 45 minutes. Subtract that number from the time you need to wake up to arrive at your bedtime.
For example, if you need to wake up at 7 a.m., work backward by 7 hours and 45 minutes to come to a bedtime of 11:15 p.m.
You can also aim for seven full 90-minute sleep cycles, which totals 9 hours of sleep. Tack on that 15 minutes to fall asleep again, and your bedtime for a 7 a.m. wake-up would be 9:45 p.m.
Naturally, you might have to bake in a bit more time if it takes you longer than 15 minutes to fall asleep.
If anything with numbers makes your head swim, the Sleep Foundation has a handy calculator to help you land on your bedtime.
Puza says being well-rested is a top priority because of all the ways sleep impacts overall health.
“I’m obsessed with getting the best sleep possible — essential for anti-aging, muscle recovery, memory and brain health, and so much more!” he said.
But how does this work for naps?
Factoring in the stages of the sleep cycle can also be helpful in determining how long to take a nap without feeling more tired when you wake up.
Thomas Michael Kilkenny, DO, director of the Institute of Sleep Medicine at Northwell Staten Island University Hospital, previously told The Post that naps should be no longer than 20 to 30 minutes.
“Sleeping longer can trigger the effects of sleep inertia. This is the feeling of continued sleepiness upon awakening,” he explained.
Sleep inertia can be felt when awakening from any stage of sleep, but it’s worse when waking up from deep sleep.
“When this happens, it is difficult to jump out of a deeper stage of sleep and become fully awake. Our brains want to continue to finish the deeper sleep before waking up,” Kilkenny added. “When awakened from lighter sleep stages, sleep inertia can be suppressed.”
Other sleep tips
In a previous video, Puza shared four musts to help with sleep. His recommendations included a magnesium roller and an eye mask.
“Block out all light at night — even ambient light around you prevents you from getting the deepest sleep possible,” he said.
