The Dark Side of Daylight Saving Time: How Twice-Yearly Clock Changes Harm Sleep and Health

It’s no secret: many people in developed countries are chronically short on sleep. More than a third of Americans fail to reach the commonly recommended minimum of seven hours a night, and similar sleep shortfalls exist across other wealthy nations. Eva Winnebeck, a chronobiologist at the University of Surrey, observes:

“Sleep deprivation abounds. People do struggle to get up. Alarm clock use is high, lack of sleep is high.”

Why Daylight Saving Time compounds the problem

Against this backdrop of chronic sleep loss, societies still deliberately shift clocks twice a year. Each spring and fall—this year, in the U.S., the autumn switch falls on November 2—many people’s internal rhythms are forced to adjust. That abrupt, artificial change can worsen sleep deprivation and desynchronize biological rhythms that evolved to follow the sun.

What experts recommend

Major sleep and circadian organizations have spoken against seasonal clock changes. The American Academy of Sleep Medicine (AASM) and several European sleep and biological-rhythm societies argue that “these seasonal time changes should be abolished,” recommending a single, stable time year-round so daily schedules align more closely with natural daylight.

Health consequences of chronic misalignment

Public-health research links persistent sleep deprivation and circadian disruption with a wide range of harms: more motor-vehicle crashes, increased risk of diabetes and obesity, higher blood pressure, greater risk-taking behavior, and elevated rates of heart attacks. For shift workers, who already face misalignment between internal clocks and daylight, these effects are particularly pronounced.

When internal biological rhythms fall out of order, physiological processes that should occur at different times can overlap; others that should coincide drift apart. Scientists believe this desynchronization may help explain higher rates of heart disease and certain cancers seen in night-shift workers.

Historical and policy context

The twice-yearly clock changes are a government policy with modern roots. The current U.S. approach stems from the Uniform Time Act of 1966; earlier, temporary wartime experiments with clock shifts aimed to save energy by moving more human activity into daylight hours. Standardized DST returned as regulators sought consistency for trains and planes crossing state lines.

Since 2025, most Americans observe Daylight Saving Time for eight months (March–November), with Standard Time lasting about four months. A few places—such as Hawaii and much of Arizona—do not observe DST.

Natural seasonal rhythms vs. artificial clock changes

Human biology does not demand a fixed day length year-round. Because the Earth’s axis is tilted, daylight hours change with the seasons—and these differences are larger at higher latitudes. Research indicates humans may be naturally set up to vary sleep duration by season. For example, a 2015 study of pre-industrial societies found people typically woke shortly before sunrise year-round and slept about three hours after sunset, staying awake longer in summer than in winter. Similar re-alignment with the sun is seen when modern people camp without artificial light.

Daylight Saving Time imposes an artificial, abrupt shift that often conflicts with these natural cues. As chronobiologist Martha Merrow of Ludwig Maximilian University of Munich puts it:

“We should be looking for ways to not use an alarm clock. Every time we use an alarm clock, we deprive ourselves of sleep.”

Evidence from time-zone edges and short-term effects

It’s difficult to pin down the exact effect of DST on any individual, but population-level research provides clues. Studies that compare outcomes across the east and west edges of the same time zone—where solar time and clock time are differently aligned—show concerning patterns. According to Elizabeth Klerman of Harvard Medical School, the western edges (where people are effectively living later relative to the sun) show higher rates of cancer, obesity, diabetes and sleep insufficiency—conditions associated with circadian disruption.

Short-term harms from the spring clock change are better documented: many studies report a rise in traffic accidents and a notable spike in heart attacks immediately after the shift to Daylight Saving Time, with reductions after returning to Standard Time.

Policy debate and alternatives

Policymakers have repeatedly debated eliminating seasonal changes. In 2019 the European Parliament voted to end the practice, but implementation stalled because countries could not agree whether to adopt permanent Standard Time or permanent DST. Some experts favor permanent Standard Time because it aligns more closely with solar time, while others emphasize flexible work schedules that let individuals and communities shift their work hours seasonally.

More radical ideas exist: a return to local solar time—so noon is when the sun is highest—and letting computers handle cross-region coordination. Historically, before railways required standardized time, local solar time was common. While such a change is logistically complex, it highlights the trade-off between coordination and biological alignment.

Conclusion

Twice-yearly clock changes are a politically entrenched practice with real biological implications. Evidence from short-term spikes in accidents and heart attacks and from longer-term geographic comparisons suggests that prolonged misalignment between clock time and solar time can harm health. Options range from keeping a single, year-round time (most sleep experts favor permanent Standard Time) to promoting flexible schedules that let people align daily routines with natural light.

Contact: letters@time.com