In the United States, the risk of cancer increased the further west one lived in the same time zone – that is, in places where social time is further from solar time, a situation similar to permanent daylight saving time.
By: Prof. Eran Tauber, head The Biological Clock Research Laboratory, University of Haifa
This past weekend, the clocks went back and we all gained an extra hour of sleep. The transition from daylight saving time to winter time is actually a return to natural and standard time, as it is called in most countries around the world – standard time. Daylight saving time was introduced at the beginning of the last century in many countries to move the clocks forward so that more daylight hours would appear in the afternoon, when people are awake and active.
Many processes in the human body exhibit a circadian rhythm or cycle. This cycle is created and managed by an internal biological system called the circadian clock. It is an internal mechanism that operates on a roughly twenty-four-hour cycle and coordinates the activity of cells, body systems, and behavior with the circadian cycle of light and darkness. The circadian clock controls almost every aspect of human physiology – sleep and wakefulness, hormone secretion, blood pressure, body temperature, and metabolism. Exposure to natural light in the morning helps synchronize the biological clock with the new day, while darkness in the evening signals the body to slow down and prepare for sleep.
When we move our clocks forward for daylight saving time, we create a gap between our biological clock and our social clock. In the morning, the sun rises later according to the clock, and we wake up before our bodies have received their biological signal to wake up. In the evening, the sun sets later, and we are exposed to more light during the hours when our bodies should already be producing melatonin, the sleep hormone. This gap causes what is known as “circadian misalignment,” and it manifests itself in fatigue, decreased concentration, disrupted sleep patterns, increased appetite, mood swings, and even an increased risk of heart disease, diabetes, and obesity.
Studies have found that the transition to daylight saving time causes significant health damage in the short term. In the United States, a 6% increase in traffic accidents and a 27% increase in heart attacks were reported in the days following the transition. A study published last September in the prestigious journal PNAS A study by Stanford researchers found that standard time is the most natural and healthiest setting for the human body. The researchers showed that the biannual switch to and from daylight saving time causes a lasting disruption to the circadian clock, potentially leading to hundreds of thousands of preventable strokes and obesity cases each year in the United States. They concluded that maintaining a fixed standard time is the healthiest choice for the public.
Additionally, there are large studies that have examined the health of people living on different sides of the same time zone. Geographically, the western side of the time zone experiences later sunrise and sunset than the official time, while the eastern side is closer to true solar time. This means that in effect, those living in the western time zone live in a constant state similar to perpetual daylight saving time, while those living in the eastern time zone live in a constant state similar to perpetual winter time. Therefore, the comparison between east and west within the same time zone effectively serves as a natural model for comparing living on perpetual daylight saving time versus living on perpetual winter time.Prof. Eran Tauber
A study published in 2017 inThe journal Cancer Epidemiology, Biomarkers & Prevention showed that in the United States, the risk of cancer increased the further west one lived in the same time zone—that is, in places where social time was further from solar time, a situation similar to permanent daylight saving time. The researchers explained that these people suffer from a chronic mismatch between their biological clock and the official time. Another study published in 2024 inCancer Research Communications Re-examined the data and found that the differences were not significant across all disease types, but the possible effect still existed in certain populations.
Switching to permanent daylight saving time may seem convenient, because it adds hours of daylight in the evenings during the summer. But from a biological perspective, this is a problematic solution. In the summer, the days are already long, and permanent daylight saving time would further delay sunrise. This means that in the winter, the sun will rise very late, sometimes only after 7:30 or 8 a.m., when children and students are already on their way to school in complete darkness. From the perspective of the circadian clock, exposure to light in the morning is essential. It signals the body that the day has begun, helps synchronize sleep and wake cycles, and improves alertness, concentration, and mood. In contrast, exposure to strong light in the evening inhibits the production of the hormone melatonin, delays the onset of sleep, and impairs its quality. These effects are not the same for the general population. People with a late chronotype, sometimes called “owls,” tend to go to bed late and wake up late, and are therefore particularly affected when the social clock moves away from the natural time of sunrise. Among teenagers, who are naturally more inclined to a nocturnal chronotype, this effect is even more acute. They have difficulty waking up in the morning, experience significant fatigue during school hours, and suffer from a decline in cognitive function and concentration. Permanent daylight saving time may therefore deepen the gap between the social clock and the biological clock precisely among the populations most vulnerable to this disruption.
In parallel with the scientific publications, in recent years many scientific societies have published official position papers calling for the abolition of daylight saving time. The American Academy of Sleep Medicine stated in 2020 that a transition to a permanent standard clock should be preferred, because it is better synchronized with the human biological clock and reduces health risks. The European Society for Sleep Research also published a similar position paper, which stated that maintaining a permanent daylight saving time would harm public health and cause prolonged disruption of the circadian rhythm. Similar positions were also expressed by the Canadian Society of Sleep Medicine and the National Association of Occupational Medicine in Australia. The growing scientific consensus is that the abolition of clock changes and a return to a permanent winter clock is the healthiest solution for individuals and society.
The overall conclusion from all the evidence is that maintaining a fixed winter time, that is, standard time, most accurately reflects the Earth's light and dark cycle and the needs of the human biological clock. Daylight saving time, whether temporary or permanent, distances us from natural coordination with light, impairs sleep and health, and leads to an increase in health risks. The transition to winter time is not just a technical change of one hour, but an important correction that returns us to the biological harmony to which humans have adapted over millions of years of evolution.
Want to understand in depth the mechanisms of the circadian clock, their impact on health and sleep, and how to synchronize your life with the body's natural rhythms?
You are invited to join the digital course “Biological Clocks: The Body’s Internal Rhythm Mechanisms.”
Students – The course will open in the second semester, details inDepartment of Evolutionary and Environmental Biology
Free learners – The course is also available on the IL Campus platform. Register (free) click here
the article Switching to winter time: Why it's worth staying on it all year round First appeared on the website Haifa University.
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