It’s time to fall back again.

The clock will strike 1 a.m. twice Sunday morning as daylight saving time once again comes to an end.

Here’s what you need to know about daylight saving time and why the U.S. changes clocks twice a year.

When does daylight saving time end?

Daylight saving time began March 10 and ends Nov. 3.

Unlike in the spring, when we lose an hour and the clocks skip the 2 o’clock hour entirely, we will gain an extra hour Sunday, with clocks jumping from 1:59 a.m. back to 1 a.m.

The sun will also start setting earlier across the U.S. as the temperatures continue to cool down and we head into the late fall and winter months.

Why is this happening?

The practice, established by the Standard Time Act in 1918, according to the U.S. Astronomical Applications Department, is an effort to extend the daylight hours we have in the summertime by pushing off sunset an extra hour.

Daylight saving time, a contested idea after it was first passed, was quickly repealed in 1919, becoming a local matter. It was re-enacted during the early days of World War II and was observed from 1942-1945, according to the department.

After the war, the implementation of daylight saving time varied from state to state until the Uniform Time Act passed in 1966, which standardized the dates of daylight saving time but allowed for local exemptions if states or localities did not want to participate.

According to the department, the standardized start and end dates have been changed throughout the years, but since 2007, daylight saving time has started on the second Sunday in March and ended on the first Sunday in November.

How long does standard time last?

Standard time across the U.S. will remain, as will earlier sunsets and darker evenings, until the spring rolls around and daylight saving time starts once again.

In 2025, daylight saving time starts on Sunday, March 9, and ends on Sunday, Nov. 2, when it’s time to repeat the process.

Are there any states that don’t observe?

Yes, Hawaii and most of Arizona do not observe daylight saving time and therefore do not change their clocks twice a year, according to the Astronomical Applications Department.

Is the U.S. the only country that does this?

No, most countries observe some version of “summer time,” according to the department. In the Northern Hemisphere, most of the countries that observe daylight saving time are in Europe and North America.

Though other countries observe a version of daylight saving time, not all do so on the same schedule as the U.S.

There are also Southern Hemisphere countries that observe some version of daylight saving time, but below the equator, the seasons are swapped, so the start and end date of their “summer time” are reversed from ours, according to the department.

According to the Pew Research Center, only about a third of all countries observe daylight saving time. At one point, about half of all countries observed the practice but no longer do.

What efforts have been made to end the practice?

In March 2022, the U.S. Senate passed the Sunshine Protection Act, which would make daylight saving time permanent year-round and usher out the era of changing our clocks.

Under the bill, Hawaii and most of Arizona would continue to observe year-round standard time.

But the bill has since stalled in the House of Representatives, meaning the U.S. will continue to observe the time change until it is passed in that chamber and then signed by the sitting president.

Almost all states have considered legislation to stay on standard or daylight saving time, and 20 states have passed bills or resolutions to implement year-round daylight saving time in the last six years, according to the National Conference of State Legislatures. But, because federal law currently does not allow for year-round daylight saving time, the states would have to wait for Congress to pass the bill in order to make the change.

What do health experts say?

Some studies suggest that using daylight saving time year-round could reduce the number of traffic accidents and the amount of crime.

But a number of experts aren’t in favor of permanent daylight saving time. That’s because the sun should reach the highest point in the sky at noon, according to sleep experts, which is known as solar time.

During standard time, people in the central time zone in the U.S. are aligned perfectly with solar time, but during daylight saving time, they are pushed further away from that clock.

The more mismatch with solar time, the higher the risk of health problems, Dr. Karin Johnson, a neurology professor at UMass Chan Medical School-Baystate who is on the board of Save Standard Time, a nonprofit that advocates for permanent standard time, told NBC News in 2022.

Sleep experts prefer the back-and-forth of the clocks to permanent daylight saving time. When people wake up in darkness, hormones like cortisol might be higher, which might make people feel sleepier, Dr. Kin Yuen, a sleep medicine specialist at the University of California, San Francisco, and a fellow at the American Academy of Sleep Medicine, said in 2022.

Then, because the sun is out later, people might go to sleep later during daylight saving time, which can delay the body’s production of melatonin.

Plus, a study from June 2022 found that people whose clock times weren’t closely aligned with the sun had 22% higher road fatality rates than those living within 30 minutes of solar time.