Apr 2nd 2025
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As clocks spring forward in the northern hemisphere, many people will be looking forward to longer, sunnier evenings—a few groggy mornings is a price they’re probably willing to pay. But a growing body of research suggests that they ought to be more cautious. The arrival of Daylight Saving Time (DST) seems to have long-lasting negative effects on human health.
The human body clock, known as the circadian rhythm, lives in a region of the brain’s hypothalamus known as the suprachiasmatic nucleus (SCN). It determines whether you feel awake or sleepy by regulating the production of melatonin, the sleepiness hormone, and cortisol, a stress hormone that promotes alertness. It is also essential for the timing of bodily functions, including the expression of genes and regulation of metabolism and mood.
What sets the body clock ticking is a person’s exposure to light, particularly the blue wavelengths that are more prevalent in the morning. Specialised cells in the retina send instructions to the SCN and, when the clock is set off correctly in the morning, sleep hormones appear at the right time in the evening and people wake up naturally at the right time the next day.
Standard time, what some may think of as winter time, more closely aligns with the natural light-dark cycles of day and night. On standard time, people get more blue early-morning light, thus setting off their body clocks properly. Not only is there less of this morning light during DST, the extra hour of light in the evenings makes things worse by interfering with melatonin production—thus delaying the onset of sleep. All this adds up to a misalignment of sleep-and-wake rhythms during DST, and it also plays havoc with the release of cortisol, ghrelin (a hunger hormone) and leptin (the satiety hormone).
A study in the Journal of Health Economics in 2019 looked at people living on opposite sides of a time zone in America. On the late Sun side (in effect an hour behind the early Sun side), on average people were sleeping 19 minutes fewer every day during DST, were 11% more likely to be overweight and obese, and their wages were 3% lower. Other studies show that the chronic effects are worse for teenagers, who are already biologically prone to being alert at night. They lose 32 minutes a night.
The sudden shift to DST itself also brings short-term health and safety risks. One study showed that, in the morning after the time shift, participants lost about 40 minutes of sleep, on average. Further problems include a spike in strokes (8%), heart attacks (24%), suicides (6%), and a slight increase in fatal traffic accidents. Moreover, researchers have noted an impact on episodes of depression, and studies in mice have tied DST to inflammatory bowel disease.
There are ways to reduce some of these health problems. Eva Winnebeck, a chronobiologist at the University of Surrey, says that sitting outside in the morning (without sunglasses) can reset the biological clock. Avoiding the glare of screens late at night will also help, as will changing your home’s lighting to warmer hues in the evening to lessen the dampening effect of bright light on the production of melatonin. There is nothing to stop you from enjoying the long evenings, of course, whether you decide to do some gardening or play golf. But perhaps consider wearing orange-tinted glasses that filter out any blue light. You might look a little strange, but can you really put a price on a good night’s sleep? ■
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This article appeared in the Science & technology section of the print edition under the headline “Is Daylight Saving Time bad for you?”
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