It happens every year, but for many of us, the clocks going back comes as a surprise.
The colder days and darker nights also means a change from British Summer Time (BST) back to Greenwich Mean Time (GMT).
What time do the clocks go back?
The clocks go back at 2am - meaning we get an extra hour to celebrate Halloween.
It also means the mornings will begin to get a bit lighter and the evenings will get darker earlier.
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The longest night of the year - or the shortest day - will be due a few days before Christmas - on December 21.
It's called the winter solstice and happens when the North Pole reaches its maximum tilt away from the sun.
When do the clocks go forward again?
The clocks will go forward by an hour again in spring 2022.
Unfortunately it will mean losing an hour in bed, but it also brings shorter nights and more light during the day.
That's something to look forward to after a long, cold autumn and winter, as the time changes to BST.
The move forward will happen on Sunday, March 27 at 1am.
If you get confused by which way the clocks change and when, try thinking of the seasons: spring, forward and autumn (or fall) backwards.
