TThe good news: you get a wonderful extra hour of sleep. The bad: It will be dark as hell in the US by late afternoon for the next few months

Daylight saving time ends this Sunday, November 3 at 2 a.m. local time, which means you’ll need to set your clocks back an hour before going to bed. Standard time will last until March 9, when we will “spring forward” again with the return of daylight saving time.

That change in spring can be harder on your body. Darker mornings and lighter evenings can disrupt your internal biological clock, making it harder to fall asleep on time for weeks or more. In fact, research has shown that the number of heart attacks and strokes increased immediately after the time change in March.

“Falling back” should be easier. But it can still take a while to adjust your sleeping habits, not to mention the disadvantages of working or exercising in the dark when there is still plenty of light. Some people with seasonal affective disorder, a form of depression usually linked to the shorter days and less sunlight in the fall and winter, may also have difficulty with this.

Some health groups, including the American Medical Association and American Academy of Sleep Medicine, have said it’s time to do away with timers and that sticking to standard time is better aligned with the sun — and human biology.

Most countries do not observe daylight saving time. For those that do – especially in Europe and North America – the date the clock is changed varies.

Two states – Arizona and Hawaii – do not change and remain on standard time.

Here’s what you need to know about the biennial ritual.

How the body responds to light

The brain has a master clock that is set by exposure to sunlight and darkness. This circadian rhythm is a cycle of approximately 24 hours that determines when we become sleepy and when we are more alert. The patterns change with age, one reason young early risers evolve into hard-to-wake teenagers.

Morning light resets the rhythm. Towards the evening, levels of a hormone called melatonin begin to rise, causing drowsiness. Too much light in the evening – that extra hour from daylight saving time – delays that peak and the cycle gets out of sync.

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

How do time changes affect sleep?

Even a change of time on the clock can throw off sleep schedules – because even though the clocks change, work and school start times remain the same.

That’s a problem because so many people are already sleep deprived. About 1 in 3 American adults sleep less than the recommended seven-plus hours per night, and more than half of American teens don’t get the recommended eight-plus hours during the week.

Sleep deprivation is linked to heart disease, cognitive decline, obesity and a host of 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 before the time change. There are ways to ease the adjustment, including more sunshine to help reset your circadian rhythm for healthy sleep.

Will the US ever get rid of the time change?

Lawmakers occasionally propose doing away with the time change altogether. The most prominent recent effort, a now-stalled bipartisan bill called the Sunshine Protection Act, proposes to make daylight saving time permanent. Health experts say lawmakers did it backwards: Standard time should be made permanent.