Twice a year most of the United States moves its clocks by an hour. The body's internal timing does not move with them, and the gap is what people experience as adjustment.
Light sets the internal clock, not the wall clock
Circadian timing is anchored primarily by light reaching the eye, particularly in the morning. That signal tells the internal clock what time it is.
Changing a clock face does not change sunrise. After a shift, the same internal timing now sits an hour away from the schedule it is expected to follow.
Realignment happens gradually as morning light arrives at the new social hour, and the internal clock drifts to meet it over subsequent days.
Spring is harder than fall
Moving clocks forward removes an hour of sleep opportunity and requires waking earlier relative to the sun, which asks the internal clock to advance.
Advancing is the more difficult direction. Human circadian rhythms tend to run slightly long, so delaying is easier than pulling the whole system earlier.
The fall change gives an hour back, but it also brings darkness earlier in the evening, which affects mood and outdoor activity for the rest of the season.
The mismatch has a name and a mechanism
Researchers use the term social jetlag for the routine gap between a person's biological timing and the schedule imposed by work, school and transport.
A clock change enlarges that gap abruptly for everyone at once, which is why the effects are visible in the days immediately following rather than spread out.
People whose natural timing already runs late experience a larger shift, since they were further from the social schedule before the change occurred.
Time zone geography compounds it
American time zones are wide, so the same clock time corresponds to noticeably different solar times at the eastern and western edges of a zone.
Residents on a zone's western edge already live with a sun that rises and sets late relative to the clock, and a spring shift moves them further in that direction.
Not all of the country participates. Some states and territories remain on standard time year-round, which removes the shift but leaves the zone geography unchanged.
Adjustment is faster with light management
Morning light exposure after the change accelerates realignment, because it supplies the signal the internal clock uses to reset.
Shifting sleep and wake times gradually across several days spreads the change rather than delivering it all on one night.
Sleep problems that persist well beyond the transition are not caused by the clock change, and continuing difficulty is worth raising with a clinician.