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Sleep researchers have increasingly focused on the concept of social jetlag, a term describing the mismatch between a person's biological sleep preferences and the schedule imposed by work, school, or social obligations, distinct from the jetlag experienced after crossing time zones. Social jetlag is measured by comparing the midpoint of a person's sleep on free days, when they can wake naturally, against the midpoint of sleep on workdays, when an alarm forces earlier waking; a difference of two or more hours is associated in multiple population studies with elevated rates of obesity, cardiovascular disease, and depressive symptoms, even after controlling for total sleep duration. The effect appears strongest among adolescents and young adults, whose biological clocks naturally shift toward later sleep and wake times during puberty, placing them in direct conflict with early school start times that were set decades ago based on adult schedules rather than adolescent chronobiology. Several school districts that have since delayed start times by even thirty to sixty minutes have reported measurable improvements in student attendance, mood, and standardized test performance, lending support to the idea that the mismatch itself, rather than sleep duration alone, drives much of the harm. Employers have been slower to respond, in part because flexible start times are harder to implement in roles requiring team coordination or customer-facing hours, and because many managers remain skeptical that a scheduling mismatch, rather than simple insufficient sleep, could meaningfully affect health outcomes. Skeptical researchers have also noted that social jetlag studies are largely observational, meaning they can identify correlation but cannot fully rule out the possibility that people prone to depression or poor health independently develop irregular sleep patterns for unrelated reasons, rather than the mismatch causing the health effects directly. Despite this uncertainty, public health bodies in several countries have begun citing social jetlag in recommendations urging schools and employers to allow more flexibility in start times where operationally feasible.