Twice a year, clocks across most of the United States shift by one hour, disrupting sleep patterns, changing daylight exposure, and triggering a measurable ripple of health and safety effects. In spring, clocks jump forward at 2:00 a.m. on the second Sunday in March, cutting a night short by one hour. In fall, clocks fall back at 2:00 a.m. on the first Sunday in November, adding an hour to the night. The spring transition tends to be harder on the body and carries more documented risks.
What Changes When the Clocks Shift
The mechanics are simple: at 2:00 a.m. local time, clocks either skip ahead to 3:00 a.m. (spring) or rewind to 1:00 a.m. (fall). Most phones and computers update automatically. The practical effect in spring is that sunrise and sunset both arrive an hour later by the clock, giving you more evening daylight but darker mornings. In fall, the reverse happens: mornings brighten earlier and evenings get dark sooner.
For 2025, daylight saving time began on March 9 and ends on November 2. In 2026, it ends on November 1. Hawaii, most of Arizona, and U.S. territories like Puerto Rico and Guam don’t observe daylight saving time at all.
How Your Body Responds
Your internal clock, which runs on roughly a 24-hour cycle, relies on light exposure to stay synchronized with the outside world. When the clock shifts by an hour overnight, your body doesn’t catch up instantly. Your brain still releases sleep-promoting hormones on the old schedule, and your stress hormones still peak at their usual biological time, even though the clock on the wall says something different.
The spring shift is particularly disruptive because you lose an hour of sleep with no warning to your biology. Research shows the average person sleeps about 40 minutes less on the Monday after springing forward compared to other nights of the year. That doesn’t sound like much, but for people who are already short on sleep, it compounds an existing deficit. Most people take several days to fully adjust, and some studies suggest subtle circadian misalignment can linger for weeks.
The fall transition is generally easier because you gain an hour, but it still reshuffles your light exposure. Darker evenings can affect mood and energy levels, especially for people sensitive to seasonal changes in daylight.
Heart and Stroke Risks
For years, early studies suggested a spike in heart attacks right after the spring transition. More recent and larger analyses have complicated that picture. A study published in JAMA Network Open examined nearly 170,000 heart attack patients and found no significant difference in heart attack rates during the spring daylight saving week compared to the week before or after. About 17.0% of cases fell during the transition week, versus 16.9% the week before and 16.7% the week after.
Stroke risk, however, does appear to climb briefly. Research presented through the American Academy of Neurology found that the overall rate of ischemic stroke (the most common type, caused by a blocked blood vessel in the brain) was 8% higher during the first two days after a daylight saving time transition. The increase was temporary, but it points to the sensitivity of the cardiovascular system to even small disruptions in sleep and circadian rhythm.
Driving Becomes More Dangerous
The spring time change is consistently linked to more fatal car crashes. A study examining the effect of changing light conditions found that fatal crashes among vehicle occupants increased by 12% in the five weeks after the spring transition. When averaged across the full period, the spring shift was associated with a 3.3% increase in weekly fatal crashes. The likely culprits are a combination of sleep deprivation, darker morning commutes, and impaired alertness.
The fall transition, by contrast, appears to have a protective effect. The same research found fatal crashes decreased by about 7.1% in the five weeks after falling back, with an overall 2.4% weekly reduction. Gaining an hour of sleep and having brighter mornings both likely contribute.
The Energy Savings Debate
Daylight saving time was originally promoted as an energy conservation measure. The idea was straightforward: more evening daylight means less need for electric lighting. In practice, the savings are negligible or nonexistent.
A study of California’s one-month extension of daylight saving time found a near-zero impact on electricity consumption. Research in Indiana, which adopted statewide daylight saving time in 2006, actually found that residential electricity demand increased by about 1%. The reason: while lighting costs dropped slightly, heating and cooling costs rose because people were awake during hotter or colder parts of the day. Some Scandinavian research has shown modest electricity savings of at least 1% in southern Norway and Sweden, but those countries have very different climates and daylight patterns than most of the U.S. The bottom line is that energy savings no longer hold up as a strong justification for the practice.
Why Sleep Experts Want It Abolished
The American Academy of Sleep Medicine has taken a firm public position: the U.S. should eliminate seasonal time changes entirely and adopt permanent standard time. Their reasoning is that standard time aligns best with human circadian biology. When the sun is closest to its highest point at noon, your internal clock and the social clock are in harmony. Permanent daylight saving time, which some lawmakers have proposed, would push sunrise later year-round, meaning dark winter mornings that conflict with school and work start times.
The AASM’s position emphasizes that the evidence supports distinct health and safety benefits from standard time, while also highlighting the harms that come from switching back and forth twice a year. The twice-yearly disruption itself, not just which time zone you’re in, is part of the problem.
How to Ease the Transition
Sleep researchers at Harvard Medical School recommend shifting your sleep schedule gradually in the days leading up to the change. For the spring transition, try going to bed and waking up 15 to 20 minutes earlier each night for three or four nights before the clocks change. For the fall transition, shift later by similar increments. This gives your internal clock time to catch up rather than forcing it to adjust all at once.
Light exposure is your most powerful tool. In the spring, getting bright light in the morning helps your body shift earlier. In the fall, morning light helps prevent your schedule from drifting too late. Avoiding screens and bright indoor lighting close to bedtime supports the process in both directions. Most people feel fully adjusted within four to seven days, but being deliberate about sleep and light can shorten that window considerably.