“Fall back” should be easier. But it can still take a while to adjust your sleep habits, not to mention the downsides of leaving work in the dark or trying to exercise while there’s still enough light. Some people with seasonal affective disorder, a type of depression usually linked to the shorter days and less sunlight in 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 do away with time switches and that sticking to standard time fits better with the sun — and human biology.

Most countries do not have summer time. For those that do – mostly in Europe and North America – the date the clocks change varies.

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

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

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 get sleepy and when we are more alert. Patterns change with age, one reason why early-arriving youths develop into hard-to-wake teenagers.

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

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

Even a one hour change in the clock can throw off sleep patterns – because even if the clock changes, work and school start times remain the same.

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

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

Some people try to prepare for a 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 sunshine to reset your circadian rhythm for healthy sleep.

Legislators sometimes suggest getting rid of 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 have it backwards – standard time should be made permanent.

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The Associated Press Health and Science Department is supported by the Howard Hughes Medical Institute’s Science and Educational Media Group. AP is solely responsible for all content.