A study reveals the potential benefits of ending clock changes for heart health and weight

Our bodies, however, may not be adjusting quite so effortlessly.

Fresh research indicates the well-worn habit of moving between standard time and daylight saving time does more than throw off our alarms. By gently disturbing the body’s internal timing, it could be pushing the likelihood of strokes, heart issues and weight gain in the wrong direction - and stopping the clock changes for good might help.

How light quietly sets your internal clock

A small area deep within the hypothalamus houses the brain’s “master” circadian clock. Working on an approximately 24-hour cycle, it coordinates almost every bodily system - including body temperature, blood pressure, appetite and hormone release.

Light is the clock’s strongest signal. Specialised cells in the eyes relay information to the brain according to daylight exposure. Early light effectively tells the body “wake up, speed up”, whereas light later in the day tends to push the clock back and slow it down.

Left to itself, this internal rhythm is not precisely 24 hours. In most adults it runs a touch longer - about 24 hours and a few minutes. That is why regular morning light is needed to pull the system back into step with the real day–night pattern.

When social time and solar time drift apart, our biology spends every day catching up - and never quite gets there.

Anything that alters when we receive light - late-night screen use, working late shifts, or a one-hour change in official time - can knock this timing out of alignment. Once the system drifts, sleep schedules, hunger cues, blood sugar regulation and even blood-vessel tone can all be influenced.

The hidden stress of changing the clocks

In theory, moving the clock by one hour in spring and autumn can seem insignificant. Many assume the main consequence is simply a drowsy Monday. A new study in the Proceedings of the National Academy of Sciences suggests that view is overly rosy.

The researchers modelled what would happen under 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

To do this, they merged fine-grained geographical details - such as latitude, sunrise times and where people live - with health data from the US Centers for Disease Control and Prevention (CDC). Their aim was to estimate how each approach affects “circadian misalignment” across a year, and what that could imply for public health.

The modelling indicated that the existing arrangement, with the twice-yearly switch, creates the largest overall disruption to circadian timing. A constant system is gentler on the body because it avoids the annual lurch forwards and backwards.

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.

These figures are projections from the model rather than observed totals. Even so, they illustrate the potential magnitude: modest shifts in daily exposure to light and in sleep timing, multiplied across millions of people, could translate into sizeable changes in cardiovascular and metabolic risk.

Why standard time beats permanent summer time

According to the study, both permanent standard time and permanent daylight saving time would ease the year-round circadian burden compared with switching in spring and autumn. But the two fixed options are not equivalent.

Standard time aligns more closely with solar time - so midday sits nearer to when the sun reaches its highest point. That keeps morning daylight earlier, which generally helps stabilise circadian rhythms.

With permanent daylight saving time, winter sunrises would arrive later. In some northern areas, many people would wake up, travel and begin work in darkness for much of the season. That pattern can delay the internal clock, reduce sleep and increase misalignment for a substantial share of the population.

Time policy	Winter morning light	Circadian strain (modelled)	Health outlook
Current clock changes	Variable, plus abrupt shifts twice a year	Greatest	More disruption, higher projected risk
Permanent daylight saving time	Sunrise later, darker mornings	Medium	Less disruption than now, but still sizeable
Permanent standard time	Sunrise earlier, more morning light	Smallest	Most favourable profile for sleep and health

Morning larks, night owls and who benefits most from clock changes and circadian misalignment

People’s body clocks are not all the same. Chronobiologists often separate “morning types” from “evening types” - commonly described as larks and owls. One Stanford Medicine analysis suggests roughly 15% of people are strongly in the morning group, while a larger proportion tends to prefer later schedules.

Morning types naturally wake earlier and become sleepy earlier. Evening types, by contrast, favour a later bedtime and a later start. For these night owls, the later sunrise associated with daylight saving time might appear, on paper, to be less harsh.

However, if school and work still begin early, a later sunrise simply increases the amount of waking time spent in darkness, reduces morning light exposure and deepens the clash between social schedules and biological timing. That mismatch is often labelled “social jet lag” - the sensation of living as though you are 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.

In daily life, the signs can be familiar: persistent fatigue, inconsistent sleep, difficulty waking on workdays, and then compensating with extra sleep at weekends. Over the years, these patterns are associated with higher rates of obesity, diabetes, depression and cardiovascular problems.

What clock policy means for the working day

Arguments about changing the clocks are often presented as a matter of preference: brighter evenings for leisure versus brighter mornings for easier starts. The new results push the discussion towards public health considerations.

When legal time drifts away from solar time, more people end up living out of step with daylight. Immune function may be dampened, reaction times can worsen and cognitive performance may slip. That matters not only for an individual’s risk of stroke or weight gain, but also for productivity at work, achievement at school and accident rates.

Permanent standard time would not solve everything. At higher latitudes, winter days would still be brief and gloomy. Even so, cutting down avoidable misalignment between alarm clocks and body clocks could reduce a constant, low-level strain on both health services and employers.

What “circadian misalignment” actually means

“Circadian misalignment” can sound theoretical, so it helps to imagine a standard weekday. Your alarm goes off at 6:30am because you need to start work early. If the sunrise is late, your internal clock may effectively still behave as though it is closer to 4:30 or 5am. Your body temperature is lower, melatonin remains high, and your heart and metabolism are not fully “switched on”.

Now picture repeating that routine five days a week, week after week, and then sleeping briefly and irregularly at weekends. That ongoing mismatch is what researchers capture when they model circadian strain. It is not a single poor night; it is a continuing gap between when the body is primed to sleep and when daily life requires wakefulness.

Over time, that gap can influence how blood vessels respond under stress, how effectively the body processes glucose and fats, and how inflammatory pathways behave - all of which play a part in cardiovascular disease and in weight regulation.

Practical ways to support your body clock

Although governments decide national time policy, individuals can limit some of the biological downsides created by current rules. Several habits can help keep the rhythm steadier:

• Spend at least 20–30 minutes outside in morning light on most days, particularly after the clocks change.
• Keep wake-up times reasonably consistent, including at weekends, to reduce social jet lag.
• Lower screen brightness and reduce strong indoor lighting one to two hours before bedtime.
• Try to eat at regular times, since unpredictable meals can also push the body clock out of sync.
• Steer clear of long naps in the evening, which can shift sleep pressure later into the night.

Even if countries eventually adopt permanent standard time, these approaches would still be useful. The difference is that they would support, rather than fight against, the overall timing of daylight. The modelling suggests that pairing supportive policy with healthier routines could measurably influence long-term rates of stroke, obesity and other chronic conditions.