Here's what happens to your body when clocks 'fall back' an hour
[October 28, 2025]
By LAURAN NEERGAARD
Plan on a glorious extra hour of sleep as most of America “falls back”
into standard time. But make sure to get outside for some morning sun,
too — it’ll help your body clock reset faster.
Daylight saving time ends at 2 a.m. local time Sunday, which means you
should set your clock back an hour before you go to bed. Standard time
will last until March 8 when we will again “spring forward” with the
return of daylight saving time.
There’s a lot of grumbling about the twice-a-year time changes. The
spring switch tends to be harder, losing that hour of sleep we allegedly
recover in the fall. But many people also mourn fall's end of daylight
saving time, when days already are getting shorter and moving the clocks
can mean less daylight after school or work for exercise or outdoor fun.
Some health groups, including the American Medical Association and
American Academy of Sleep Medicine, have long urged adopting standard
time year-round.
New research from Stanford University agrees, finding that switching
back-and-forth is the worst option for our health. The study showed
sticking with either time option would be a bit healthier, but they
found permanent standard time is slightly better — because it aligns
more with the sun and human biology, what’s called our circadian rhythm.
“The best way to think about it is as if the central clock were like a
conductor of an orchestra and each of the organs were a different
instrument,” said Jamie Zeitzer, who co-directs Stanford’s Center for
Sleep and Circadian Sciences.
More light in the morning and less at night is key to keeping that
rhythm on schedule — all the instruments in sync. When the clock is
regularly disrupted by time changes or other reasons, he said each of
the body's organ systems, such as the immune system or metabolism, “just
works a little less well."
Most countries do not observe daylight saving time. For those that do —
mostly in North America and Europe — the date that clocks are changed
varies. In the U.S., Arizona and Hawaii don’t change and stay on
standard time.
Here’s what to know about the twice-yearly ritual.
How the body reacts to light
The brain has a master clock that is set by exposure to sunlight and
darkness. This circadian rhythm is a roughly 24-hour cycle that
determines when we become sleepy and when we’re more alert. The patterns
change with age, one reason that early-to-rise youngsters evolve into
hard-to-wake teens.
Morning light resets the rhythm. By evening, levels of a hormone called
melatonin begin to surge, triggering drowsiness. Too much light in the
evening — whether from later time outdoors doing daylight saving time or
from artificial light like computer screens — delays that surge and the
cycle gets out of sync.
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The sun rises, seen through morning mist and silhouetted maple
leaves, as trees turn to fall foliage colors, Tuesday, Oct. 7, 2025,
in Auburn, N.H. (AP Photo/Charles Krupa, File)
 And that circadian clock affects
more than sleep, also influencing things like heart rate, blood
pressure, stress hormones and metabolism.
How do time changes affect sleep?
Even an hour change on the clock can throw off sleep schedules
because even though the clocks change, work and school start times
stay the same.
The spring change to daylight saving time can be a little rougher as
darker mornings and lighter evenings make it harder to fall asleep
on time. Those first few days have been linked to increases in car
crashes and even an uptick in heart attacks.
Some people with seasonal affective disorder, a type of depression
usually linked to the shorter days and less sunlight of fall and
winter, may struggle too.
Many people easily adjust, like how they recover from jet lag after
traveling. But a time change can add pressure on shift workers whose
schedules already are out of sync with the sun, or those regularly
sleep-deprived for other reasons.
About 1 in 3 U.S. adults sleep less than the recommended seven-plus
hours nightly, and more than half of U.S. teens don’t get the
recommended eight-plus hours on weeknights.
Chronic sleep deprivation is linked to heart disease, cognitive
decline, obesity and numerous other problems.
How to prepare for the time change
In both fall and spring, changing bedtimes by as little as 15
minutes a night in the days before the change can help ease into it.
But sunshine in the morning is critical to helping reset your
circadian rhythm for healthful sleep. If you can’t get outdoors, sit
by windows.
Will the US ever get rid of the time change?
In Congress, a bill named the Sunshine Protection Act that proposes
making daylight saving time permanent has stalled in recent years.
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