The clock on the wall jumps an hour overnight. The clock in your body doesn't. It keeps its own time, set mainly by when you see daylight, and it moves by roughly an hour a day at most. So for a few days after the change, your body is running an hour off the new schedule: a small, self-inflicted jet lag that everyone in your time zone gets at once.
When the clocks go forward in spring, you lose an hour. Your body wants to stay up later by the new clock and resists the earlier alarm, which is why the Monday after tends to be rough. This is usually the harder change.
When the clocks go back in autumn, you gain an hour. It's gentler, but many people wake an hour early for a few days and feel sleepy early in the evening, and the darker afternoons arrive at once.
Why Shifting Early Helps
The American Academy of Sleep Medicine suggests moving your bedtime 15 to 20 minutes a night for a few nights before the change. Small steps are easy for your body clock to follow, so by the Sunday most of the hour is already behind you. The planner moves your wake time by the same amount, which keeps your time in bed the same.
Light does the real work. Morning daylight pulls your clock earlier, and evening light pushes it later. That's why the plan asks for morning light before a spring change and evening light before an autumn one. About 20 minutes outside is plenty on most days; never look directly at the sun. Read more in the morning sunlight guide.
After the Change
Keep your wake time steady on the new clock, even at the weekend, and get outside soon after waking. Keep your last coffee well before your new bedtime: for one average cup, that's roughly five hours for an average metaboliser. The caffeine cutoff calculator works out your own number.