Yet our bodies may not be coping so easily.
New research suggests that the familiar ritual of switching between standard time and daylight saving time does more than mess with our alarm clocks. By subtly disrupting our body’s internal rhythms, it may be nudging the risk of strokes, heart problems and weight gain in the wrong direction — and a permanent end to clock changes could help.
How light quietly controls your internal clock
Deep in the brain, in a small region of the hypothalamus, sits our master circadian clock. It runs on roughly a 24-hour cycle and keeps almost every system in the body in sync — from body temperature and blood pressure to appetite and hormone release.
This clock takes its main cue from light. Specialised cells in the eye send signals to the brain based on daylight exposure. Morning light signals “wake up, speed up”, while evening light tends to delay and slow the clock.
Our internal rhythm, left on its own, doesn’t run at exactly 24 hours. For most adults, it’s slightly longer, around 24 hours and a few minutes. That means we need regular morning light to pull the clock back into line with the actual day–night cycle.
When social time and solar time drift apart, our biology spends every day catching up — and never quite gets there.
Anything that shifts the timing of light exposure — late-night screens, late work shifts, or a one-hour jump in the official time — can misalign that clock. Once the rhythm is off, sleep timing, hunger signals, blood sugar control and even vascular tone can be affected.
The hidden stress of changing the clocks
On paper, changing the time by one hour in spring and autumn looks trivial. Many people assume the main effect is a groggy Monday morning. The new study, published in the Proceedings of the National Academy of Sciences, suggests that assumption is far too optimistic.
Researchers modelled the impact of three different time policies across the United States:
- Permanent standard time (no seasonal change)
- Permanent daylight saving time (no seasonal change)
- The current system with two clock changes per year
They combined detailed geographical data — including latitude, sunrise times and population distribution — with health statistics from the US Centers for Disease Control and Prevention (CDC). The goal was to estimate how each time regime affects “circadian misalignment” over the year, and what that might mean for population health.
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The modelling suggested that the current system, with the biannual switch, produces the greatest circadian disruption overall. A fixed time system is kinder to the body, as it stops the clock from lurching forward and back each year.
The study estimates that permanent standard time could prevent up to 300,000 strokes and cut obesity cases by around 2.6 million annually in the US.
Those numbers are model-based projections, not direct counts. Still, they give a sense of scale: small changes in daily light and sleep patterns, applied across millions of people, may produce surprisingly large shifts in cardiovascular and metabolic risk.
Why standard time beats permanent summer time
The study found that both permanent standard time and permanent daylight saving time would reduce yearly circadian strain compared with springing forward and falling back. Yet they are not equal.
Standard time more closely tracks actual solar time — meaning noon is closer to when the sun is at its highest point. That keeps morning light earlier, which tends to stabilise circadian rhythms.
Under permanent daylight saving time, sunrise in winter would move later. In some northern regions, people might wake, commute and start work in darkness for much of the season. That could delay their internal clock, shorten sleep and increase misalignment for large parts of the population.
| Time policy | Morning light in winter | Circadian strain (modelled) | Health outlook |
|---|---|---|---|
| Current clock changes | Varies, plus abrupt shifts twice a year | Highest | More disruption, higher projected risk |
| Permanent daylight saving time | Later sunrise, darker mornings | Moderate | Less disruption than now, but still significant |
| Permanent standard time | Earlier sunrise, more morning light | Lowest | Most favourable profile for sleep and health |
Morning larks, night owls and who benefits most
Not everyone’s body clock ticks at the same pace. Chronobiologists distinguish between “morning types” and “evening types” — often called larks and owls. A Stanford Medicine analysis suggests around 15% of people fall strongly into the morning group, while a larger share leans towards later schedules.
Morning types naturally wake early and feel sleepy earlier. Evening types prefer a late bedtime and later start. For these night owls, a later sunrise, as seen with daylight saving time, might feel less punishing on paper.
Yet when work and school still start early, later sunrise simply means more time awake in the dark, less morning light, and more conflict between social and biological time. That conflict is sometimes called “social jet lag” — the feeling of being permanently in the wrong time zone.
Across the whole population, the models suggest permanent standard time best reduces this social jet lag and the strain it causes.
Signs of this mismatch show up in everyday life: chronic tiredness, irregular sleep, struggling to wake on workdays, and a strong rebound of sleep at weekends. Over years, these patterns relate to higher rates of obesity, diabetes, depression and cardiovascular problems.
What clock policy means for the working day
The debate over clock changes is often framed as a lifestyle preference: lighter evenings for leisure versus lighter mornings for easier starts. The new findings shift that argument towards public health.
When legal time moves away from solar time, more of the population lives out of sync with daylight. Immune function may weaken, reaction times drop and cognitive performance can slip. That has implications not only for individual risk of stroke or weight gain, but also for workplace productivity, school achievement and accident rates.
Permanent standard time will not fix every problem. Winter days in higher latitudes will still be short and dim. Yet reducing avoidable misalignment between alarm clocks and body clocks could ease a constant, low-level pressure on health systems and employers.
What “circadian misalignment” actually means
The phrase “circadian misalignment” can sound abstract, so it helps to picture a typical weekday. Your alarm rings at 6:30am because work starts early. Under a late sunrise schedule, your internal clock might still think it is closer to 4:30 or 5am. Your body temperature is low, melatonin levels are still high, and your heart and metabolism have not fully “switched on”.
Now imagine forcing that pattern five days a week, every week, with a short, irregular sleep at weekends. That chronic mismatch is what researchers model when they talk about circadian strain. It is not one bad night; it is a persistent gap between when your body wants to sleep and when your life demands that you stay awake.
Over time, this gap affects how blood vessels respond to stress, how efficiently the body uses glucose and fats, and how inflammatory processes behave — all factors in cardiovascular disease and weight regulation.
Practical ways to support your body clock
While national time policy sits with governments, individuals can reduce some of the biological fallout from current rules. A few habits support a more stable rhythm:
- Get outside for at least 20–30 minutes of morning light most days, especially after the clocks change.
- Keep wake-up times fairly consistent, even at weekends, to reduce social jet lag.
- Dim screens and strong indoor lights one to two hours before bed.
- Aim for regular mealtimes, as erratic eating can also push the body clock out of sync.
- Avoid long evening naps, which can shift sleep pressure later into the night.
If countries eventually move to permanent standard time, these strategies would still matter. Yet they would work with, rather than against, the overall timing of daylight. The new modelling suggests that combination — supportive policies and healthier routines — could have a measurable effect on rates of stroke, obesity and other chronic conditions over the long term.