Circadian rhythm: the master clock

Your suprachiasmatic nucleus runs on a ~24.2-hour internal clock. Light hitting your retina (especially morning light) entrains this clock to the actual 24-hour day. Without light cues, you'd drift later by ~12 minutes per day.

Sleeping 2 hours later on weekends and waking 2 hours later is biologically equivalent to flying east two time zones. Monday morning is essentially perpetual jet lag — and it's linked to obesity, mood disorders, and metabolic syndrome.