Every year, as clocks move forward in spring and back in autumn, billions experience what seems like a harmless ritual — a one-hour shift in time. Yet scientists increasingly warn that this annual disruption does far more than confuse schedules. It unsettles the body’s biological rhythms, alters sleep patterns, heightens cardiovascular risks, and even affects mood stability. The twice-yearly clock change, they say, has become one of the world’s largest involuntary experiments on human health.

The Biological Misalignment Behind the Clock Shift

At the heart of the issue is the body’s circadian rhythm — an internal 24-hour clock finely tuned to light and darkness. This biological system dictates when we sleep, wake, eat, and perform at our best. When daylight saving time (DST) shifts the clock forward, it creates an artificial mismatch between environmental light cues and our internal timekeeping.

In spring, the one-hour “loss” delays exposure to morning sunlight while extending light late into the evening. The body’s production of melatonin — the hormone that induces sleep — is suppressed later at night, making it harder to fall asleep. Cortisol, the hormone responsible for alertness, is released later in the morning, leaving people groggy and fatigued.

Scientists describe this as a form of “social jet lag,” where internal time and societal time are misaligned. Unlike travel-related jet lag, however, this shift affects entire populations simultaneously and can persist for months. The result is cumulative sleep deprivation, slower reaction times, and disruptions to immune and metabolic functions — even among those who feel only mildly affected.

Why the Health Risks Rise After the Spring Transition

The springtime clock change has repeatedly been linked to sharp increases in heart attacks, strokes, and accidents in the days following the transition. The primary cause, researchers say, is the sudden deprivation of sleep and hormonal imbalance caused by disrupted circadian alignment.

Data from multiple countries — including the United States, Sweden, Germany, and Brazil — show a consistent rise in cardiovascular events immediately after the clocks move forward. Studies indicate a roughly 4 percent increase in heart attacks during the first few days of daylight saving time, along with a measurable uptick in strokes.

The mechanism is straightforward: less sleep and higher stress hormones lead to increased blood pressure, inflammation, and impaired blood-vessel function. These factors together raise the risk of acute cardiovascular incidents, particularly in individuals already vulnerable to heart disease.

Beyond hospitalizations, there is a documented rise in fatal road accidents following the spring shift. Fatigued drivers, combined with darker early mornings, create a dangerous combination on roads. Researchers estimate that the week after the clock change sees a 6 percent increase in fatal crashes — a small percentage that, across entire nations, translates into hundreds of preventable deaths each year.

Mental Health and Emotional Toll

While physical risks attract the most attention, the mental health impact of daylight saving time is equally concerning. Studies show that the sudden disruption to sleep and daylight exposure patterns can worsen anxiety, depression, and mood instability.

Autumn’s backward shift, though offering an extra hour in bed, coincides with darker evenings and shorter days — conditions known to trigger seasonal affective disorder (SAD). The lack of morning sunlight diminishes serotonin levels, a neurotransmitter crucial for mood balance. In Denmark, long-term data revealed an 11 percent rise in major depressive episodes during the 10 weeks following the autumn clock change.

Sleep researchers point out that chronic disruption of the circadian system, even by an hour, weakens emotional regulation and cognitive performance. People become more irritable, less focused, and prone to stress. For those already dealing with mood disorders, the shift can act as a physiological stressor that exacerbates symptoms.

Broader Metabolic and Cognitive Effects

The consequences of daylight saving time extend into metabolism and cognitive performance. Sleep loss, even modest in duration, triggers hormonal changes that increase hunger and cravings for high-calorie foods. Over time, such patterns contribute to weight gain and higher risks of diabetes and obesity.

Cognitive effects are also notable. Reaction times slow, memory retention declines, and concentration suffers in the days following the clock change. The result is lower workplace productivity, higher accident risk in industrial settings, and greater fatigue among shift workers — a group already vulnerable to chronic sleep misalignment.

Scientists warn that while these effects may appear temporary, they accumulate over years. Repeated circadian misalignment, even by small increments, accelerates wear on the cardiovascular and metabolic systems, creating a slow-moving but measurable impact on population health.

Why the Entire Population Feels the Strain

One of the most striking features of daylight saving time is that its health costs are distributed across nearly everyone. While each person may lose just an hour of sleep, when applied to entire nations, the cumulative burden is enormous.

Researchers liken the biannual clock shift to a public health stress test: an artificial change imposed on millions of biological systems at once. Some adapt quickly, while others — particularly older adults, children, and those with existing conditions — struggle for weeks. Even after adjustment, many continue to operate out of sync with their internal clocks for the duration of the daylight saving period.

Geography compounds the problem. People living on the western edges of time zones experience a longer natural misalignment, as their local sunrise and sunset occur later relative to the official clock. For them, daylight saving time further delays natural light cues, adding another layer of physiological strain.

Growing evidence of health consequences has reignited debate over whether daylight saving time should be abolished altogether. Advocates for change argue that the original rationale — conserving energy during wartime — no longer holds. Modern lighting and work habits have made energy savings negligible, while the human costs have become increasingly clear.

Public opinion is divided. Some prefer the longer evenings of daylight saving time, associating them with leisure and outdoor activity. Others, particularly sleep scientists, support adopting permanent standard time, which aligns more closely with natural sunlight patterns and human biology.

Most experts agree that the current biannual switching is the worst option. The European Parliament voted in 2019 to end clock changes, but implementation stalled amid disagreements among member states over which time to adopt. In the United States, the “Sunshine Protection Act,” which would make daylight saving time permanent, remains mired in political debate despite bipartisan interest.

The Biological Case for Standard Time

Circadian researchers overwhelmingly favour permanent standard time over permanent daylight saving time. The reasoning is simple: standard time aligns morning light exposure with human physiology. Morning sunlight is essential for resetting internal clocks, improving alertness, and regulating hormones.

Permanent daylight saving time, by contrast, would mean darker mornings and delayed light exposure for half the year — conditions associated with greater depression risk, reduced school performance, and higher accident rates. According to scientists, human health depends more on early light exposure than late-evening brightness.

In essence, the argument is not against enjoying daylight but against manipulating it artificially. As Professor David Ray, a leading circadian biologist, notes, “Every time we shift the clock, we are asking people to live out of sync with their biology. Even an hour matters when multiplied across a population.”

The Path Forward

The persistence of daylight saving time illustrates the tension between tradition, policy, and science. What began as a wartime measure to conserve coal has evolved into a global norm — one that now appears to undermine public health more than it benefits economic activity.

As more studies confirm the biological and social costs of biannual clock shifts, pressure is mounting on governments to act. Until then, millions will continue to adjust — reluctantly and biologically — to a ritual that no longer fits the rhythms of modern life.

The next time the clock moves forward or backward, it’s not just the numbers on a dial that shift. It’s an entire network of hormones, neurons, and metabolic systems trying to catch up — a reminder that the human body keeps its own time, regardless of what the clock says.

(Adapted from BBC.com)