Clocks across most of the United States fell back one hour overnight, marking the end of daylight saving time for 2026 and returning the country to standard time until March. Arizona and Hawaii, which do not observe the switch, were unaffected.
Unlike the spring change — which steals an hour of sleep — the autumn shift technically hands one back. But sleep researchers say the body's adjustment is not that simple. A University of Surrey analysis of the clock-change literature found that deep sleep, not just total sleep time, dips in the days following either transition, even when the extra hour should in theory help.
What the research shows
A Stanford-led study published in the Proceedings of the National Academy of Sciences modeled three policies — the current twice-yearly switch, permanent standard time, and permanent daylight saving time — and found that switching twice a year is the worst option for circadian health of the three. The researchers estimated that eliminating the biannual change in favor of permanent standard time could meaningfully reduce rates of obesity and stroke nationally, while cautioning that their model relies on idealized light-exposure patterns rather than people's actual day-to-day behavior.
Other research has linked the clock changes to short-term spikes in cardiovascular events: one Michigan hospital reported a 24% jump in heart attacks the Monday after the spring change, and Finnish researchers found an 8% rise in ischemic strokes in the two days following it. A 2020 University of Colorado Boulder study in Current Biology also tied the spring shift to a 6% increase in car crashes during the following work week, based on more than 700,000 fatal-crash records.
A policy debate that keeps resurfacing
The American Academy of Sleep Medicine has formally recommended the US adopt permanent standard time, arguing it aligns better with the body's natural clock than permanent daylight saving time would. Congress has repeatedly floated bills to end the twice-yearly switch altogether — a current proposal, H.R. 7378, would instead shift clocks by just 30 minutes, which an AASM spokesperson said would be gentler on the body than a full hour of permanent daylight saving time. None of the competing proposals has passed.
For now, the practical advice from sleep specialists is the same every November: get morning light exposure in the days after the change, keep a consistent bedtime, and expect a few groggy mornings before your circadian rhythm catches up to the new clock.