Sleep needs vary considerably across the lifespan. Infants require the most, around 14 to 17 hours per day, while toddlers need 11 to 14 hours including naps. School-age children should get 9 to 11 hours, and teenagers 8 to 10 hours. Most adults need 7 to 9 hours per night for optimal functioning, and seniors can usually manage on 7 to 8 hours, although sleep becomes lighter and more fragmented with age.
Beyond total hours, two physiological processes govern when and how much we sleep. The first, called sleep pressure, builds steadily during wakefulness and is driven largely by the accumulation of adenosine, a neurotransmitter that promotes sleepiness. The second, the circadian rhythm, is the body's roughly 24-hour internal clock that cycles alertness and sleepiness across the day.
These two processes, known as Process S (sleep pressure) and Process C (circadian rhythm), interact to determine sleep timing and quality. Caffeine works by blocking adenosine receptors, temporarily reducing perceived sleepiness, and with a half-life of about 5 to 6 hours, its effects linger well into the evening, which is why experts often recommend stopping caffeine 8 to 10 hours before bed. When people chronically fall short of their sleep needs, they accumulate sleep debt, which can only be partially repaid even with weekend catch-up sleep; weekday deficits leave lasting health costs.