There is no universal optimal bedtime, but the evidence consistently points to three factors that matter most: circadian alignment (going to bed when your biological clock is ready), adequate duration (seven to nine hours for most adults), and regularity (consistent bed and wake times). Irregular sleep timing — even when total sleep hours appear sufficient — is independently associated with poorer cardiometabolic and mental health outcomes. Chronotype (morning vs evening preference) is biologically real, partly genetic, and shifts predictably across the lifespan. Forcing sleep against your chronotype carries measurable health costs. The concept of ‘social jet lag’ — the mismatch between biological and social time — is an emerging risk factor in its own right.