Recommended sleep duration is not constant across life. Consensus recommendations place newborns at 14–17 hours per 24 hours, school-age children at 9–11 hours, adolescents at 8–10 hours, and adults at 7–9 hours. The decline is steepest in the first years of life and then flattens: total sleep time falls sharply through childhood, changes little through adulthood, and in older adults the total may stay near seven hours while the composition shifts, with less slow-wave sleep and more fragmented nights.
The adolescent case is the one with the clearest mismatch between biology and schedule. At puberty, the circadian timing system shifts later by roughly one to three hours. Melatonin release begins later in the evening, and the internal signal for sleep onset moves past 11 p.m. for many teenagers. The homeostatic drive still builds during the day, but the circadian gate opens later, so an adolescent who goes to bed at 10 p.m. may lie awake not from defiance but because the biological night has not started.
School start times interact badly with this shift. A start time of 7:30 a.m. requires waking near 6:30 a.m., which truncates the final cycles of the night. Because REM sleep is concentrated in the last third of a night's sleep, an early wake time removes a disproportionate share of REM. The result is a chronic shortfall in a stage that supports emotional processing and procedural memory, layered on top of a total sleep time below the recommended 8–10 hours. Later start times have been associated with longer total sleep and better attendance, which is consistent with the phase delay being biological rather than a matter of discipline.