"Get eight hours" is the sleep advice everyone repeats, but no official body actually says it. The National Sleep Foundation and the American Academy of Sleep Medicine both give ranges that change with age. If you're getting enough hours and still waking up tired, look at how well and how regularly you sleep.
Official ranges by age
The NSF's current guidance: newborns 0–3 months need 14–17 hours a day, infants 4–11 months need 12–15, toddlers 11–14, preschoolers 10–13 including naps, school-age children 9–11, and teenagers 8–10. Adults 18–64 need 7–9 hours; past 65 the range drops to 7–8, and the AASM's floor for adults is "at least 7."
The adult range spans two hours, so someone who feels great on 7 and someone who needs 9 are both normal. "Everyone needs eight hours" has no official source. It's the middle of the range repeated until it sounded like a rule.
Some people really do need less. Natural short sleepers carry specific gene variants and do fine on 4–6 hours, but research puts them at under 3% of the population. Everyone else who keeps cutting sleep builds up a measurable drop in cognitive performance, often without feeling it.
Three signs when the hours aren't the problem
Do you fall asleep when you shouldn't? A mild dip in the mid-afternoon is normal. Fighting sleep in meetings, over paperwork or as a car passenger, or nodding off the moment things get quiet, is a classic sign of too little or poor-quality sleep. Feeling sleepy isn't the test. Falling asleep where you shouldn't is.
Does your day run on caffeine? If you need two or three coffees to function, you're using caffeine to cover a sleep debt. Caffeine doesn't reduce the debt, it only pushes the tiredness back, and an afternoon cup can hurt that night's sleep, which makes the next day worse.
How much do you sleep in on weekends? Six hours on weeknights and sleeping until noon on Saturday is called social jetlag, and it's a warning sign on its own. If your weekend wake time is more than two hours later than your workday one, you're short on sleep during the week. Sleeping in helps a bit, but it doesn't make up the difference.
What your tracker's sleep stages can tell you
Trackers split the night into light, deep and REM sleep using movement and heart rate. That's an estimate, not the full sleep study a clinic would run. They're most useful for trends: two weeks of deep sleep clearly below your own normal matters more than one night that shows 8%. For adults, deep sleep usually makes up about 13–23% of the night and REM about 20–25%, and one odd night isn't worth acting on.
Pay attention if you're sleeping enough hours on a regular schedule and still get sleepy during the day, snore with pauses in your breathing, or wake up with headaches. That can point to a sleep disorder such as sleep apnea, which more sleep won't fix, so it's worth seeing a sleep clinic. If you hit all three signs above without those symptoms, fix your schedule first, and see a doctor if you're still tired after that.
Sources: National Sleep Foundation duration recommendations (2026), AASM adult sleep consensus, SEQ Toolkit's age breakdown, Qualia's evidence-based guide.