Sleep requirements are not fixed — they change dramatically from infancy through old age. The National Sleep Foundation and American Academy of Sleep Medicine both publish evidence-based guidelines that show how sleep need evolves. Infants require up to 17 hours for brain development; teens need 8–10 hours during a period of intense neural pruning; most adults function optimally on 7–9 hours; older adults tend to sleep lighter and often slightly less, though 7–8 hours remains ideal.
The biggest myth in sleep science is that you can train yourself to need less sleep. Short sleepers (those who genuinely function on 5–6 hours) carry a genetic mutation that is extremely rare. For almost everyone else, chronic short sleep accumulates as sleep debt, with measurable cognitive deficits that the person often does not perceive.
Chronic sleep deprivation is often invisible to the person experiencing it — the brain adapts to feeling tired and stops recognising it as abnormal. Key signs include: needing an alarm to wake up most days (natural waking suggests adequate sleep), falling asleep within minutes of lying down (normal onset is 10–20 minutes), strong cravings for caffeine to function, mood irritability or emotional reactivity, difficulty concentrating or making decisions, and getting sick frequently (sleep debt suppresses immune function).
A joint consensus statement by the American Academy of Sleep Medicine and the Sleep Research Society (Watson et al., 2015, Sleep) recommends that adults sleep 7 or more hours per night on a regular basis; regularly sleeping less is associated with health problems including weight gain, high blood pressure, heart disease and depression.