Timing is everything

On Sunday, March 30, 2025, the clocks in Germany will spring forward for daylight saving time. While we all adjust our watches, phones, and computers, what does this shift really do to our bodies? Experts from three different Max Planck Institutes shed light on this question.

To the point:

• The body's central internal clock is located in the suprachiasmatic nucleus (SCN) of the brain. Along with other internal clocks—such as those in the heart and liver—it regulates various bodily functions, including sleep, digestion, and immune responses.
• Following the time change, traffic accidents increase by approximately 6 percent, and the risk of heart attacks rises by 24 percent in the days immediately after.
• When the body's internal clock is disrupted, it can lead to serious health issues.
• People with sleep disorders are especially affected by seasonal clock changes.
• Scientists at the Max Planck Institute suggest abolishing the time change, as it disrupts the body's internal clock.

Text: Nora Lessing

Deep within our bodies, molecular clocks silently tick, orchestrating the essential processes that keep us alive. They govern how alert we feel, whether our digestion is functioning smoothly, and how prepared our immune system is to defend against threats. The master clock, known as the suprachiasmatic nucleus (SCN), resides in the brain. As chronobiologist Manuel Spitschan from the Max Planck Institute for Biological Cybernetics in Tübingen explains, “The SCN is connected to the eye.” He studies how light affects the fine-tuning of our internal clocks. “When light hits melanopsin-containing ganglion cells in the retina, it converts the light signal into nerve impulses, which are then sent to the SCN, adjusting our internal timing.” This process helps regulate hormone release, shifting our state of alertness. For example, when light enters our eyes, we wake up, and our focus sharpens.

Our internal clocks don’t always sync with social time

Social time, driven by human convention, governs the rhythm of wristwatches, computers, and mobile phones. But our body’s internal clock doesn’t pay much attention to these artificial timekeepers. "For our body, solar time is and remains the decisive signal," emphasizes Manuel Spitschan. This misalignment between light signals and socially imposed time creates inevitable conflicts." When the social clock changes and I suddenly have to wake up an hour earlier, my internal clock isn't adjusted to these new conditions," explains Spitschan. The result is a form of social jet lag. The time it takes for our internal clock to realign with the new schedule depends on our individual biology, but on average, it takes anywhere from one to four days to adjust.

Anyone who's travelled across multiple time zones is familiar with the discomfort of jet lag. Although the body is adaptable and will eventually adjust, you might think that a single hour of change wouldn’t have much of an impact. But several studies show that the consequences of shifting the clocks can be far more significant. For instance, just after the clocks are set forward, road accidents spike by about 6 percent. This is likely due to more fatigued, distracted drivers on the roads. At the same time, there’s also a noticeable uptick in strokes. Researchers at the University of Colorado have found that the day after the clocks spring forward, the rate of heart attacks rises by about 24 percent. But why does this happen?

"The SCN is the most important, but it’s not the only internal clock," explains Pieterjan Dierickx from the Max Planck Institute for Heart and Lung Research. Dierickx is studying how cardiac metabolism shifts throughout the day and the consequences of those changes. "The liver, pancreas, and heart all receive signals from the SCN. However, each of these organs also has its own internal clock, which it uses to regulate various rhythmic processes within the body."

These peripheral clocks communicate throughout the day, sending signals in a precise sequence and intensity. For example, depending on the rhythm of the heart’s clock, the heart muscle may be particularly efficient at pumping blood during certain times—or more vulnerable to stress and pathogens at others. The early morning hours seem to be especially critical. When a heart attack occurs during this time, it tends to be more severe than usual, both in humans and in mice.

More heart attacks after the time change

"The exact mechanisms behind the increase in heart attacks right after the time change are still unclear," says Pieterjan Dierickx. "However, it’s highly likely that the disruption of the internal clock due to the time shift plays a significant role." Evidence from animal studies supports this theory. For example, mice subjected to frequent, long-distance jet lag—where researchers advance the light cycle by six hours every few days—experience serious health issues over time. "The complications are severe: heart rate problems, irregular heart rate variability, altered blood sugar levels, and increased susceptibility to weight gain," says Dierickx. "These are all risk factors that can lead to significant heart problems and even heart failure."

The impact on those with sleep disorders

The time change particularly affects people with sleep disorders. Matthias Knop, a neurologist at the Max Planck Institute of Psychiatry in Munich, studies the health consequences of disrupted sleep-wake rhythms in humans. Many conditions—such as depression, dementia, and Parkinson's disease—are known to worsen with sleep disturbances, creating a double burden for patients. However, not all patients struggling with their internal clocks are necessarily ill.

"Quite often, people come to us who wish they could fall asleep anytime, wake up refreshed, and head to work full of energy," Knop says. "But if you’re used to going to bed at 3 a.m. on Thursday and then try to switch your rhythm instantly on Sunday, it’s just not going to work."

Everyone has a different ability to adjust to changes in rhythm. "If I push it too far, I may not be as productive the next day. But as long as I accept that, it’s usually not a major issue." People with chronic sleep disorders or illnesses, however, are more affected by the clock change. "Healthy individuals can usually cope—often not immediately, but at least within a few days," says Knop. From a sleep medicine perspective, the key is not to stress about it. "Many sleep problems arise simply from focusing on the issue too much."

The case against daylight saving time

Cardiologist Pieterjan Dierickx has a piece of advice for those struggling after the time change: avoid caffeine and other stimulants in the days that follow. "These are bad for heart health and only add to the confusion of the internal clock." Dierickx, along with his colleague Manuel Spitschan, believes the time change is a self-inflicted problem. "The time shift challenges the internal clock unnecessarily," says Dierickx. "Physiologically, it would be far more sensible to stick with standard time, commonly known as winter time, year-round."