The spring and fall clock changes are small on paper and produce effects visible in population data. The reason is that they move one timekeeper and not the other.
Two clocks, only one of which moves
Social time is set by agreement and can be shifted by legislation. The internal timing system is set largely by light and cannot be shifted by decree.
Moving clocks forward means alarms ring an hour earlier in internal terms while sunrise has not moved, so waking happens at a point the body treats as night.
Realignment happens gradually as light exposure does its work, which is why the disruption lasts longer than the single lost hour would suggest.
The spring change is harder than the fall
Advancing the schedule requires falling asleep earlier than the body is ready to, and the internal system resists advances more than delays.
The fall change asks for the easier direction, and although it disturbs early waking for some people, it does not compress sleep the way spring does.
This asymmetry is consistent with what is observed when people travel across time zones in each direction.
Effects appear where sleep loss matters
Patterns following the spring change have been examined in areas where a modest reduction in alertness across a population becomes visible, such as traffic incidents.
The individual effect is small, and it is the aggregation across many people that makes it detectable at all.
That distinction matters: it describes a population signal rather than a prediction about how any particular person's Monday will go.
Standing time zones do the same thing quietly
Within a single wide time zone, clock time and solar time diverge substantially between the eastern and western edges.
People near a western edge experience later sunrises and sunsets on the same clock, which shifts their internal timing relative to their schedule year-round.
The clock change is a sharp version of an effect that geography already produces continuously and invisibly.
What eases the transition
Shifting bedtime and waking in small increments over the preceding days lets the adjustment happen gradually rather than all at once.
Getting outdoor light soon after waking in the days after a spring change supports the advance the schedule is asking for.
Anyone whose sleep problems persist well beyond a week or two, at any time of year, should raise it with a doctor rather than attribute it to the calendar.