Can daylight saving time make you ill?
Research shows that the spring transition to daylight saving time disrupts sleep and temporarily coincides with more heart attacks and fatal road accidents. That increase is short-lived.
The spring transition to daylight saving time, when the clocks move forward by one hour, is linked to more heart attacks and fatal road accidents in the days immediately after. These are associations found when large populations are studied, not proven cause and effect, but they keep showing up across multiple analyses.
The effect on sleep is the most directly demonstrated. After the clocks spring forward, you sleep less, sleep worse, and feel drowsier during the day. That effect is greatest if you are naturally a night owl. The autumn transition has the opposite effect: people who set the clocks back seem to recover temporarily better, the number of deaths falls shortly afterwards, and workplace accidents decrease as well.
Animal studies show why these kinds of effects are biologically plausible. A disrupted day-night rhythm enlarges the left chamber of the heart and impairs its pumping function. Those animal findings have been rated with varying degrees of certainty by the researchers themselves, but they apply to animals, not directly to people.
In the area of mental health, no clear effect has been found: the clock change does not demonstrably lead to more or fewer psychological complaints. The total number of steps taken per day does not change after the clock change either, although the timing of when you are active does shift, towards the morning or the evening depending on the season.
The risk of daylight saving time therefore lies mainly in the transition week itself, not across the full summer half of the year. Permanent daylight saving time is, in studies of large populations, actually associated with fewer deaths and fewer road accidents compared with permanent winter time. Anyone who is vulnerable to sleep deprivation or heart and vascular problems would do well to pay extra attention to rest and routine during the first week after the spring transition.
This answer draws on four combined summaries of existing research, one of which also includes a statistical summary of animal studies. The researchers themselves rated the certainty of the animal findings as follows: the evidence for heart enlargement is of high certainty, for larger heart muscle cells of moderate certainty, for reduced pumping function and disrupted blood vessel function of low certainty, and for other outcomes of very low certainty. The findings in people (heart attacks, road accidents, sleep) come from studies of large populations that measure associations but cannot prove cause and effect. The effects are short-lived and occur around the transition week itself.