Twice a year most of the United States moves its clocks, and the following week produces a familiar dragging feeling. The mechanism is a mismatch between two timekeeping systems that normally agree.
The body clock runs on light, not clocks
A cluster of cells in the hypothalamus sets the daily rhythm for temperature, hormone release and alertness. It runs on a period close to but not exactly twenty-four hours.
Specialized light-sensitive cells in the retina report ambient light to that cluster, which uses the signal to reset itself each day. Morning light advances the clock; evening light delays it.
A wall clock changing has no direct effect on this system. Only the light schedule and behavior that follows the wall clock reach it.
The spring shift compresses the night
Moving clocks forward removes an hour of sleep opportunity immediately and then requires waking an hour earlier relative to the sun for weeks afterward.
The internal clock has not moved. Waking now happens closer to the body's temperature minimum, the point at which sleep pressure release is least complete and grogginess is deepest.
Because the sun rises later relative to the new clock, morning light arrives later too, which weakens the very signal needed to advance the rhythm.
Re-entrainment takes days, not hours
The circadian system shifts gradually, moving a limited amount per day. A one-hour displacement therefore resolves over several days rather than overnight.
Different rhythms shift at different rates. Sleep timing may adjust before core temperature and hormone rhythms catch up, leaving the system temporarily out of internal alignment.
That internal misalignment, rather than the lost hour alone, is what people register as feeling off through the following week.
Geography changes the size of the effect
Within a single American time zone, sunrise can differ substantially between the eastern and western edges. People on the western edge already live with clock time running ahead of solar time.
The spring shift adds to that offset, so the same national change lands harder in some places than others.
Latitude matters as well, since seasonal swing in day length is far larger in Maine or Montana than in southern Texas.
Who feels it most
Later chronotypes, who naturally fall asleep and wake late, are pushed further from their preferred timing by the spring shift and have the most ground to make up.
Adolescents sit disproportionately in that group for developmental reasons, which is one reason school start times are debated alongside clock policy.
Persistent difficulty sleeping that continues well past a transition week is a separate matter and is worth raising with a clinician rather than attributing to the clock.