Americans change clocks twice a year and the effects last longer than the hour involved. The reason is that the internal clock is set by light rather than by legislation.
The body clock tracks light, not clocks
Timing in the body is governed by a cycle that runs slightly off twenty-four hours and is corrected daily by light reaching specialized receptors in the eye.
Morning light advances that cycle and evening light delays it, so the timing of exposure rather than the total amount does most of the work.
Changing what the clock reads does not change when the sun rises, which means the correction signal stays where it was while the schedule moves around it.
Spring costs more than fall
The spring transition requires waking an hour earlier relative to the internal cycle, which is the harder direction because the underlying rhythm already runs long.
Delaying is generally easier than advancing, which is the same asymmetry that makes westward travel less punishing than eastward travel of similar distance.
The spring change also moves light later in the morning for several weeks, weakening the very signal that would help the adjustment along.
Adjustment is gradual, not immediate
The internal clock shifts by a limited amount each day, so a one-hour move takes days rather than a single night to settle.
During that period, sleep timing, alertness and appetite cues sit slightly out of alignment with the schedule the calendar imposes.
People with rigid obligations early in the morning absorb the full cost, while those with flexible start times can let the adjustment happen on its own terms.
Chronotype determines who suffers
Individuals differ substantially in their natural timing, and later chronotypes already run a mismatch between preferred sleep timing and required wake times.
A transition that pushes wake time earlier relative to the body clock deepens that existing mismatch, which is why the same change is trivial for some people and rough for others.
Adolescents are notably affected, since biological timing shifts later during those years while school start times generally do not.
Why the policy debate is about light placement
Arguments over permanent standard time versus permanent daylight time are arguments about whether light lands in the morning or the evening year-round.
Sleep researchers have generally favored the option placing more light in the morning, on the reasoning that morning light is what anchors the daily cycle.
Persistent sleep problems that outlast a transition by weeks are worth raising with a clinician, since a chronic pattern is a different matter from a seasonal adjustment.