When clocks spring forward one hour for daylight saving time, your body loses an hour of sleep and your internal clock falls out of sync with the new schedule. That mismatch between your circadian rhythm and the clock on the wall triggers a cascade of short-term effects: groggier mornings, slower productivity, a modest bump in stroke risk, and a adjustment period that can last up to a week. The shift also challenges some long-held assumptions about why we change the clocks in the first place.
Your Internal Clock Loses Its Anchor
Your body runs on a roughly 24-hour internal cycle tuned to light and darkness. When the clock jumps ahead, sunrise and sunset suddenly arrive an hour later on your internal schedule, but your alarm goes off at the same time. The result is something like a mild, one-hour jet lag. Your brain still wants to be asleep when your alarm rings, and it still wants to be awake when you’re trying to fall asleep that night.
For most people, circadian rhythms reset within a few days to a week, according to the University of Rochester Medical Center. But during that window, the effects are real and measurable. Studies on U.S. miners found the spring time change cuts sleep by about 40 minutes on the first night, not the full hour you’d expect, because people were already slightly sleep-deprived beforehand and the body partially compensates.
Productivity Dips, Especially in the Morning
The lost sleep shows up clearly in how people work. In the two weeks following the spring transition, researchers tracking activity on GitHub (a platform where software developers log their work) found significant drops in output during early morning hours, between 8 and 10 a.m. Workers were starting slower, though many made up for it later in the day. At the daily level, the productivity dip lasted about two days before returning to baseline.
There’s also the cyberloafing problem. On the Monday after the spring change, Google searches for entertainment-related content spike in cities that observe daylight saving time compared to the Monday before. People are at their desks, but they’re browsing instead of working. Workplace safety also declines in the days following the transition, with studies documenting increases in on-the-job injuries and traffic accidents.
The Stroke Risk Is Small but Real
A Finnish study published in the journal Sleep Medicine found that hospitalizations for ischemic stroke increased by 8% during the first two days after the spring transition. The effect was statistically significant, though relatively modest in absolute terms. The likely mechanism is the disruption to sleep and circadian timing, which can temporarily affect blood pressure regulation and blood vessel function.
Heart attacks have received even more attention. Earlier, smaller studies suggested a spike in cardiac events on the Monday after the spring change. But a larger analysis from Duke University, covering nearly 170,000 patients over a decade, found no significant increase. During the week of the spring transition, 17.0% of heart attack cases occurred, compared to 16.9% the week before and 16.7% the week after. Those differences were not statistically meaningful. The finding, published in JAMA Network Open, suggests the heart attack link may have been overstated by studies with smaller sample sizes.
Mood and Seasonal Depression
For most people, the return of longer daylight hours in spring is a mood boost. The National Institute of Mental Health notes that many people feel better in spring as daylight increases, particularly those who experienced the “winter blues” or winter-pattern seasonal affective disorder.
A smaller group has the opposite experience. Summer-pattern seasonal affective disorder, though less common, can begin in spring and bring insomnia, poor appetite, restlessness, anxiety, and agitation. People with a history of this pattern can benefit from starting treatment before spring arrives, rather than waiting for symptoms to set in.
The Energy Savings Myth
Daylight saving time was originally justified as a way to save energy, the idea being that shifting an hour of daylight into the evening would reduce the need for artificial lighting. Modern data tells a different story. A landmark study published in The Review of Economics and Statistics examined what happened when Indiana adopted statewide daylight saving time, creating a natural experiment. Residential electricity demand actually increased by 1%, and the effect grew larger later in the DST period, reaching 2% to 4% in October.
The reason is straightforward: while DST does reduce electricity used for lighting, it increases electricity used for heating and cooling. People come home to warmer afternoons in summer and darker, colder mornings in spring and fall, and the extra air conditioning and heating outweigh the lighting savings. The researchers concluded that “the long-standing rationale for DST is questionable” and that the policy may have the opposite of its intended effect.
How to Ease the Transition
Since the adjustment takes a few days to a week, the goal is to shrink the gap between your internal clock and the new schedule as quickly as possible. The most effective tool is morning light. Getting outside or sitting near a bright window shortly after waking helps signal your brain that the day has started, pulling your circadian rhythm forward to match the new time.
In the days before the spring change, shifting your bedtime 15 to 20 minutes earlier each night can reduce the shock. Even two or three nights of gradual adjustment means you’re only losing 20 or 30 minutes instead of a full hour. Avoid sleeping in on the Sunday morning after the change, tempting as it is. Waking at your normal time (by the new clock) and getting light exposure locks in the reset faster. Caffeine can help with the morning grogginess, but cutting it off by early afternoon keeps it from interfering with the earlier bedtime your body needs.
The people most affected tend to be those who are already sleep-deprived, natural night owls whose circadian rhythms run late, and anyone with a health condition sensitive to sleep disruption. For these groups, the gradual approach in the days before the transition is especially worthwhile.