Experts outline how to find the exact amount of sleep you need

Experts say most adults can pinpoint the exact amount of sleep they need by observing their body’s natural rhythms, timing sleep around 90‑minute cycles and temporarily removing external time cues such as alarms and caffeine. The approach, described by psychologists and sleep physicians, is intended to reveal the amount of nightly sleep that leaves a person feeling refreshed and alert without reliance on stimulants.

Health specialists generally recommend seven to nine hours of sleep per night for adults, but about one in three adults report regularly getting less. Chronic short sleep and prolonged oversleeping have both been associated with a range of health problems, including cardiovascular disease, diabetes, weight gain, cognitive decline and increased mortality, according to the experts interviewed.

The practical method described in interviews with clinicians centers on two tools: sleep cycles and a short self‑directed period the experts call a "sleep vacation." Sleep cycles last about 90 minutes and alternate through light sleep, slow‑wave (deep) sleep and REM (rapid eye movement) sleep. Most adults require five to six full cycles a night, which translates to roughly 7.5 to nine hours of sleep. Waking at the end of a cycle, rather than in the middle, often reduces morning grogginess and can help a person identify their optimal sleep length.

A sleep vacation involves allowing oneself to go to bed and wake up without an alarm for about two weeks. During the first few days, people tend to repay accumulated sleep debt and often experience REM rebound, when the brain increases REM sleep to recover from prior deprivation. That initial rebound can cause daytime grogginess. After the initial adjustment, clinicians say, natural sleep patterns should stabilize and make it possible to observe how many hours of sleep feel restorative.

"When you have the luxury of taking a vacation that isn't jam-packed with activities, you can really tune into your body," said Dr. Leah Kaylor, a licensed clinical psychologist practicing in Virginia. "Without the pressure of an alarm clock, you can let your body fall asleep when it feels right and wake up naturally."

Dr. Sajad Zalzala, chief medical officer of a telehealth company, outlined sleep quality targets clinicians use when assessing restorative sleep: at least 120 minutes in REM, at least 100 minutes in slow‑wave sleep and a sleep efficiency of 90 percent or more, meaning at least 90 percent of time in bed is spent asleep. When those thresholds are not met, longer time in bed may reflect poor sleep quality or underlying conditions, rather than a true need for more sleep.

Both too little and too much sleep have been linked to cognitive decline. A 2017 analysis from the Framingham Heart Study followed more than 2,400 older adults for a decade and found that people reporting more than nine hours of sleep per night were about twice as likely to develop dementia as those who slept six to nine hours. MRI scans in that cohort showed smaller brain volumes among long sleepers, which the researchers said may indicate accelerated brain aging. Separately, a UK longitudinal study of nearly 8,000 people found that sleeping six hours or fewer in midlife was associated with higher risk of dementia decades later: roughly a 22 percent increased risk at age 50 and a 37 percent increased risk at age 60, compared with those who slept seven hours.

Experts caution that long sleep often signals poor sleep quality or underlying illness, including sleep apnea, depression or other chronic conditions, and that treating those conditions can restore normal sleep needs. Short sleep can accelerate neurodegenerative processes by disrupting the brain’s nightly clearance of metabolic waste and the consolidation of short‑term memories into long‑term storage.

Because many people cannot take extended sleep vacations, clinicians advise a mixed approach of self‑experimenting and objective tracking. Reducing or avoiding caffeine for several days before testing sleep is recommended because caffeine can disrupt the body’s internal clock and hormone release. Dr. Kaylor suggested tapering caffeine intake three to five days ahead of a self‑test, ideally limiting consumption to about 100 milligrams per day, equivalent to one cup of coffee.

Wearable sleep trackers can supplement self observation. Devices such as wrist or ring trackers estimate time spent in different sleep stages and overall sleep efficiency. Clinicians say combining tracker data with a person’s subjective daytime alertness, mood and cognitive performance over several days provides the clearest picture of ideal sleep duration.

Short and long sleep have also been tied to shorter lifespans in some studies. Researchers have estimated that chronic poor sleep may shave several years from life expectancy, with some analyses suggesting larger effects for women than men. Experts emphasize that establishing consistent, high‑quality sleep across adulthood is a modifiable behavior that can support cardiovascular and metabolic health as well as cognitive function.

The researchers and clinicians interviewed recommended starting with small, practical steps: aim for consistent bedtime and wake times, reduce evening caffeine and screens, allow a brief period for natural sleep patterns to emerge where possible, and consult a clinician if excessive sleepiness or prolonged long sleep persists. When symptoms such as loud snoring, gasping during sleep, persistent daytime sleepiness or rapidly changing sleep needs appear, evaluation for sleep disorders or other medical conditions is advised.