The National Heart, Lung, and Blood Institute (NHLBI) recommends that most adults get 7–9 hours of sleep per night. Adults 65 and older may do well with 7–8 hours; teens (13–18) need 8–10 hours; school-age children (6–12) need 9–12 hours.
Sleep needs are partly genetic. Some people function well on 6 hours while others need 10 to feel fully restored. The real measure is how you feel after waking — not the number itself.
Consistently sleeping less than 7 hours builds up a "sleep debt" that is hard to repay short-term. Weekend catch-up sleep cannot fully undo a week of accumulated fatigue.
Signs your sleep is sufficient: you wake naturally without an alarm, feel refreshed, don't doze off unintentionally during the day, and maintain stable mood and focus.