Most Americans will gain an extra hour of sleep this weekend as daylight saving time ends, but the return to standard time will also bring darker afternoons.
Experts recommend getting outside for morning sunlight to help the body's internal clock adjust more quickly to the time change.
Daylight saving time will end at 2am local time on Sunday, requiring people to set their clocks back one hour before going to bed. Standard time will remain in effect until March 14, when clocks will move forward again as daylight saving time resumes.
The twice-yearly clock change has long drawn criticism in the United States. An AP-NORC poll conducted last year found that only 12% of American adults supported the practice, while nearly half opposed it. US lawmakers revisited the issue over the summer.
Many people find the spring transition more difficult because they lose an hour of sleep. However, others dislike the end of daylight saving time in autumn, as shorter days and earlier sunsets leave less daylight for exercise and outdoor activities after school or work.
The poll found that, if given a choice, most Americans would prefer to retain the additional hour of evening daylight.
Health organisations, including the American Medical Association and the American Academy of Sleep Medicine, have long advocated keeping standard time throughout the year. They argue that standard time aligns more closely with the sun's natural cycle, particularly during winter when sunrise occurs later, and with the body's biological clock, known as the circadian rhythm.
Brighter mornings and reduced light exposure at night help maintain this rhythm, making it easier to wake up on time and maintain healthy sleep patterns.
Most countries do not observe daylight saving time. Among those that do, mainly in North America and Europe, the dates for changing clocks vary. In the United States, Hawaii and most of Arizona remain on standard time throughout the year.
How the body responds to light
The brain has an internal master clock regulated by exposure to sunlight and darkness. This circadian rhythm follows a roughly 24-hour cycle that influences when people feel sleepy and when they are most alert.
These patterns change with age, helping explain why children who wake up early may become teenagers who struggle to get out of bed in the morning.
Morning sunlight plays a crucial role in synchronising the body's internal clock. In the evening, levels of melatonin, a hormone that promotes sleepiness, begin to rise.
Excessive light exposure at night, whether from spending time outdoors later during daylight saving time or from artificial sources such as computer screens, can delay the release of melatonin and disrupt the sleep cycle.
The circadian rhythm also influences other bodily functions, including heart rate, blood pressure, stress hormones and metabolism.
How clock changes affect sleep
Even a one-hour shift can disrupt sleeping patterns because work and school schedules generally remain unchanged despite the change in time.
Following the spring transition, darker mornings and brighter evenings can make it more difficult to fall asleep at the appropriate time. The first few days after the change have been associated with an increase in road crashes and a rise in heart attacks.
During autumn and winter, some people experience seasonal affective disorder, a form of depression commonly associated with shorter days and reduced sunlight.
While many people adjust to the time change relatively easily, much as they recover from jet lag after travelling, the transition can be particularly challenging for shift workers whose schedules are already misaligned with natural daylight and for people who regularly get insufficient sleep.
About one in three US adults sleeps fewer than the recommended seven hours a night, while more than half of American teenagers fail to get the recommended eight hours of sleep on school nights.
Chronic sleep deprivation is associated with heart disease, cognitive decline, obesity and numerous other health problems.
How to prepare for the time change
Experts suggest gradually adjusting bedtime by as little as 15 minutes each night in the days leading up to the clock change, both in autumn and spring, to make the transition easier.
Morning sunlight is particularly important for resetting the circadian rhythm and supporting healthy sleep. People who cannot go outdoors can sit near windows to get exposure to natural light.
Will the US end the clock changes?
The debate over ending the twice-yearly time change resurfaces almost every year. The United States previously experimented with permanent daylight saving time in the 1970s, but the measure lasted less than a year before public opposition led to its reversal.
The latest effort came in July, when the US House of Representatives overwhelmingly passed the Sunshine Protection Act, which would make daylight saving time permanent.
President Donald Trump supports the proposal and has been pressing reluctant senators to back it. He even posted the cellphone number of Arkansas Senator Tom Cotton, who opposes the measure, on social media.