Most adults need 7 to 9 hours of sleep per night. The CDC and the American Academy of Sleep Medicine both recommend at least 7 hours for anyone aged 18 to 60, while adults 65 and older do well with 7 to 8 hours. These aren’t aspirational numbers. They reflect the amount of sleep your body requires to maintain normal cognitive function, metabolic health, and cardiovascular performance.
Recommendations by Age Group
The National Sleep Foundation’s expert panel reviewed thousands of studies to arrive at these ranges:
- Young adults (18 to 25): 7 to 9 hours
- Adults (26 to 64): 7 to 9 hours
- Older adults (65+): 7 to 8 hours
The slight narrowing for older adults reflects changes in sleep architecture with age, not a reduced need for rest. If you’re over 65 and consistently sleeping 6 hours, that’s still too little.
What Your Brain Does During Sleep
Sleep isn’t passive. Your brain runs a cleaning cycle that only works properly when you’re unconscious. During deep sleep (the third stage of non-REM sleep), the spaces between brain cells physically expand, allowing cerebrospinal fluid to flow more freely and flush out metabolic waste. This system clears out proteins like beta-amyloid and tau, both of which accumulate during waking hours and are linked to cognitive decline.
This flushing action peaks during slow-wave deep sleep, when large pulses of cerebrospinal fluid surge through the brain roughly every 20 seconds. During waking hours, that same fluid barely moves. The difference is dramatic, and it only happens when you’re getting enough total sleep to cycle through the deep stages multiple times per night.
A full sleep cycle takes about 90 to 110 minutes and includes light sleep, deep sleep, and REM sleep. In a healthy adult, deep sleep and REM sleep each account for about 25% of total sleep time. Seven hours gives you roughly four full cycles. Nine hours gives you five or six. Cut your sleep to five hours and you lose entire cycles, particularly the REM-heavy ones that come later in the night.
What Happens When You Sleep Too Little
Regularly sleeping 6 hours or fewer raises your risk of cardiovascular disease. A large meta-analysis found that people sleeping 5 to 6 hours per night had a 9% higher risk of cardiovascular events compared to those sleeping 7 to 8 hours. That number sounds modest, but it compounds over years and stacks on top of other risk factors like diet and activity level. Both short sleep (under 6 hours) and long sleep (over 8 hours) are associated with increased mortality from cardiovascular disease and cancer.
The cognitive effects are more immediate and surprisingly severe. Staying awake for 17 hours produces impairment similar to a blood alcohol concentration of 0.05%, according to NIOSH. At 24 hours without sleep, impairment reaches the equivalent of 0.10%, which is above the legal driving limit in every U.S. state. You don’t need to pull an all-nighter to feel this. Chronic short sleep, even just shaving an hour or two off each night, accumulates into measurable cognitive deficits within days.
Can You Catch Up on Weekends?
Somewhat, but the window is narrower than most people think. A large cross-sectional study found that sleeping about 45 minutes to 1 hour extra on weekends (compared to weekday sleep) was associated with lower insulin resistance, a key marker of metabolic health. For people sleeping less than 6 hours on weekdays, that one extra hour of weekend sleep was tied to a significant reduction in metabolic risk.
Here’s the catch: sleeping 2 or more hours extra on weekends was actually associated with an 88% increase in the risk of severe insulin resistance. The relationship is U-shaped. A little extra weekend sleep helps. A lot of it signals a pattern of deprivation and overcorrection that your body handles poorly.
And when it comes to cognitive performance, recovery is even harder. In controlled lab studies, participants restricted to 5 hours of sleep per night for a week showed accumulated deficits in reaction time, alertness, and mood. Even after a full 10-hour recovery night, they didn’t return to baseline. Multiple studies confirm that one or two nights of extended sleep cannot fully reverse a week of restriction. The debt is real, and the repayment schedule is slow.
Can Some People Thrive on Less Sleep?
A very small number of people carry a rare genetic mutation that allows them to function well on about 6 hours of sleep instead of 8. The most studied version involves a change in the DEC2 gene, which increases production of a brain chemical that promotes wakefulness. These individuals genuinely need less sleep. They aren’t toughing it out or relying on caffeine.
The prevalence of this mutation is extremely low. If you think you’re one of these natural short sleepers, consider that most people who believe they function fine on 5 to 6 hours have simply adapted to feeling impaired. Chronic sleep deprivation blunts your ability to perceive how tired you actually are, even as your reaction times and decision-making measurably worsen.
How to Tell If You’re Getting Enough
The simplest test: do you need an alarm to wake up? If you consistently can’t wake at the time you need without one, you’re either going to bed too late or not getting enough total sleep. Other signs of insufficient sleep include needing caffeine to feel alert before mid-morning, falling asleep within minutes of lying down (healthy sleep onset takes about 10 to 20 minutes), and feeling drowsy during afternoon meetings or while driving.
Your ideal number within the 7 to 9 hour range is personal. Some adults genuinely feel best at 7 hours, others at 8.5. The most reliable way to find your number is to spend two weeks going to bed early enough to wake naturally without an alarm, then note how many hours you consistently sleep once the initial “catch-up” period passes. That stabilized number is your target.