The spring transition to daylight saving time is experienced across most of the United States as one lost hour. The lasting part is the change in when daylight arrives relative to the clock.
Two separate things happen at once
The immediate effect is acute sleep loss, since the night is an hour shorter and most people do not compensate by going to bed earlier.
The persistent effect is a shift in the relationship between social schedules and daylight, which continues for months rather than resolving over a weekend.
Those two effects are often conflated, which is why the transition is discussed as a one-day event when the harder part is structural.
Light is the main timing signal
The internal clock is set primarily by light reaching specialized receptors in the eye, with morning light advancing the clock and evening light delaying it.
After the spring change, morning departures happen in darkness for weeks, so the strongest advancing signal is reduced exactly when the schedule demands an earlier start.
Evening light simultaneously extends later on the clock, adding a delaying signal. The two changes push in opposite directions from what the schedule requires.
Social schedules do not shift with the body
Work start times, school bells and appointments move instantly with the clock, while the body clock adjusts gradually and at different rates in different people.
The resulting mismatch between internal timing and required timing is what researchers describe as social jetlag, and the spring change increases it for most people.
Later chronotypes are affected more, since they were already waking earlier than their internal timing preferred before the clocks moved.
Consequences show up in aggregate data
Population-level patterns in the days after the spring transition have been examined repeatedly, generally showing short-term increases in sleepiness-related events.
The signal is a population effect rather than an individual prediction, which is the appropriate way to read it: risk shifts slightly for many people at once.
The fall transition, which adds an hour, produces a different and generally smaller disturbance, because acute sleep loss is not part of it.
What actually eases the transition
Shifting bedtime and wake time earlier in small steps across the preceding days spreads the adjustment rather than concentrating it into one night.
Getting outdoor light soon after waking supplies the advancing signal directly, and outdoor light is far brighter than indoor lighting even under cloud.
Sleepiness that persists for weeks after the change, or daytime sleepiness that is present year-round, points to something other than the clock and warrants a clinical evaluation.