The good news: you’ll get a glorious extra hour of sleep. The Bad: It’s going to be pitch black into the late afternoon for the next few months in the US

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Daylight saving time ends on Sunday at 2am local time, which means you should turn your clocks back an hour before going to bed. Standard Time will last until March 9th, when we will “forward” again with the return of summer time.

This spring change can be tougher on your body. Darker mornings and lighter evenings can mess with your internal clock, making it harder to fall asleep on time for weeks or more. Studies have even found an increase in heart attacks and strokes immediately after the time change in March.

“Fall back” should be easier. But it can take some time to adjust your sleeping habits, not to mention the downsides of leaving your workplace in the dark or trying to exercise while there’s still enough light. Some people with seasonal affective disorder, a type of depression typically associated with the shorter days and less sunlight of fall and winter, may also struggle.

Some health groups, including the American Medical Association and the American Academy of Sleep Medicine, have said it’s time to ditch the time switches and that keeping standard hours better aligns with the sun — and human biology.

Most countries do not observe daylight saving time. For those that do – mostly in Europe and North America – the date on which the clocks are changed varies.

Two states – Arizona and Hawaii – are not changing and remain on standard time.

Here’s what you need to know about the twice-yearly ritual.

How the body reacts to light

The brain has a master clock that is set by exposure to sunlight and darkness. This circadian rhythm is a roughly 24-hour cycle that determines when we become sleepy and when we are more alert. Patterns change with age, which is why early adopters become hard-to-wake teenagers.

The morning light resets the rhythm. Towards evening, levels of a hormone called melatonin begin to rise, triggering sleepiness. Too much light in the evening – that extra hour from daylight saving time – delays this growth and the cycle becomes out of sync.

And that circadian clock affects more than sleep, also influencing things like heart rate, blood pressure, stress hormones and metabolism.

How do time changes affect sleep?

Even a clock change can throw off your sleep schedule—because even though the clocks change, work and school start times stay the same.

This is a problem because so many people are already sleep deprived. About 1 in 3 US adults sleep less than the recommended seven hours a night, and more than half of US teenagers do not get more than the recommended eight hours on weeknights.

Lack of sleep is linked to heart disease, cognitive decline, obesity and numerous other problems.

How to prepare for the time change

Some people try to prepare for the shock of the time change by changing their bedtimes little by little in the days leading up to the change. There are ways to ease the adjustment, including getting more sunlight to help reset your circadian rhythm for healthy sleep.

Will the US ever get rid of the time change?

Lawmakers occasionally propose getting rid of the time change altogether. The most prominent recent attempt, a now-stalled bipartisan bill called Sun Protection Actproposes that summer time be permanent. Health experts say lawmakers have it back – standard time should be permanent.

Dairy farmer Aubrey Jarrell of Kentwood, Louisiana, is not a fan of the time change. He said it interfered with his cows’ strict milking routine and caused them stress “and stress on a dairy cow is not good”. There is a drop in milk production until they adjust, he said.

“Keep the time the same – whatever it is – but keep it the same,” he said.

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