How much sleep do you actually need?
Adults are generally recommended 7 or more hours per night; teenagers need 8 to 10 hours.
Sleep need is a number, not a vibe. The commonly cited figures are 7 or more hours per night for adults and 8 to 10 hours for teenagers. These are the baselines you measure everything else against: if you routinely land below them, no amount of weekend catch-up fully repays the debt.
Sleep debt accumulates quietly. One short night here and there is manageable, but a pattern of 5–6 hour nights shifts your baseline alertness, reaction time and mood before you consciously notice. The practical test is simple: if you fall asleep within minutes of your head hitting the pillow every night, you are probably carrying debt.
Age changes the requirement, but the structure does not. Whether you need 7 hours or 10, your night still cycles through non-REM and REM stages roughly every 90 minutes. Cutting the night short does not compress these cycles — it truncates them, usually at the end, where much of the REM sleep sits.
- Adults: generally recommended 7 or more hours per night.
- Teenagers: need 8 to 10 hours per night.
- A full non-REM/REM cycle lasts about 90 minutes; plan wake-ups in 90-minute blocks if you must shorten sleep.
- Chronic short sleep shows up as daytime sleepiness, irritability and difficulty concentrating.
Why the last coffee of the day matters more than the first
Caffeine has a half-life of about 5 to 6 hours, so an afternoon coffee can still affect night sleep.
Caffeine does not disappear when the cup is empty. It has a half-life of about 5 to 6 hours, meaning that a 200 mg coffee drunk at 4 pm still leaves roughly 100 mg circulating at 9–10 pm, and about 50 mg at 2–3 am. That residual dose is enough to delay sleep onset and shorten deep sleep even if you fall asleep anyway.
The cut-off depends on your bedtime. Working backwards from a 11 pm lights-out, the last caffeine should be no later than about 3–5 pm — earlier if you are sensitive or if you sleep poorly already. Tea, cola, energy drinks and dark chocolate all count toward the same budget.
Alcohol is the other common saboteur. It may help you fall asleep faster, but it fragments the second half of the night and suppresses REM. Screens are the third: blue-enriched light in the evening suppresses melatonin and delays sleep onset, so the practical cut-off for phones, tablets and laptops is roughly 60–90 minutes before bed.
- Last caffeine: about 5–6 hours before bed — no later than 3–5 pm for an 11 pm bedtime.
- Screens off: 60–90 minutes before bed to avoid suppressing melatonin.
- Alcohol: avoid within 3 hours of bed; it fragments sleep and cuts REM.
- Vigorous exercise: finish a few hours before bed; regular activity still improves sleep quality.