It's that time of year again when the clocks go back and people up and down the country celebrate an extra hour in bed - unless you're a parent of young children of course. 

This year the clocks will revert to Greenwich Mean Time (GMT) at 2am tomorrow (Sunday, October 25). 

Then, in early spring - on March 28, 2021 - the clocks will be put back forward again. 

But why do we change the clocks twice every year and where does the tradition come from?

Why do we change the clocks?

The clocks are put back every year heading into winter to effectively 'correct' British Summer Time (BST). 

Sometimes called Daylight Saving Time (DST), this is the period in summer when the clocks go forward by one hour, meaning we get up ‘earlier’ and see more sunlight.

In October, the clocks then go back again for the winter months.

Where does the tradition come from?

The idea of DST is believed to have first been proposed by British-born New Zealand entomologist and astronomer, George Hudson, in 1895.

Hudson’s shift work meant he could use his leisure time to collect insects and led him to value after-hours daylight.

In 1895, Hudson presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift. This proposal gained considerable interest in Christchurch, leading him to follow the idea up in an 1898 paper.

British builder and outdoorsman William Willet has also been credited with the invention of DST, who campaigned tirelessly for the idea to be introduced.

Willet proposed that the clocks should be advanced by 80 minutes in four incremental steps during April and reversed the same way during September.

Doing so would mean the evenings would remain light for longer, increasing daylight recreation time and also saving £2.5 million in lighting costs.

After his campaign, British Summer Time was established by the Summer Time Act 1916, beginning on May 21 and ending on October 1.

The German Empire and Austria-Hungary organised the first nationwide implementation of Daylight Saving Time on April 30, 1916 as a way to conserve coal during wartime.

Britain and most of its allies followed suit, while Russia waited until the following year and the US adopted DST in 1918.

Most places abandoned it just after the war ended, apart from Canada, the UK, France, Ireland and the US.

It grew in popularity again during World War Two and was widely adopted in the US and Europe from the 1970s as a result of the energy crisis.

Which countries use Daylight Saving Time?

Most areas in Europe and North America, including Canada and Mexico, still observe DST, but it is generally not observed in countries close to the equator, where sunrise and sunset times don’t not vary enough to justify it.

Some countries only observe DST in some regions, with only parts of Australia, New Zealand, Fiji and Samoa, observing it, while Asia and Africa do not follow it at all.