Seasonal time changes do not influence the risk of myocardial infarction.

The life of all living species is based on the day-night cycle associated with the Earth’s rotation. Humans are no exception to this rule, as the efficiency of several physiological processes essential to the body’s functioning requires optimal synchronization with the light/dark cycle. One of the best examples is the cycle governing glucose metabolism: during the day, the cessation of melatonin secretion caused by the onset of sunlight allows the pancreas to produce insulin in response to carbohydrate ingestion. The subsequent uptake of glucose from the bloodstream is used by cells to support our energy needs during waking hours. In the evening, sunset induces melatonin secretion (to promote sleep) and interferes with insulin secretion, which reduces sugar absorption by cells and facilitates the transition to using fat as the primary energy source during rest (see our article on this topic).
 
 
Social clock
 
Humans, however, differ from other animals in that they possess another type of clock, known as the “social” clock, which serves to structure most of our daily activities, whether it be work, school, mealtimes, or even our leisure time and interpersonal interactions. This social clock is somewhat different from the sunrise/sunset cycle, insofar as we sleep approximately 8 hours per day and our periods of wakefulness are therefore necessarily longer than the total duration of sunlight. For the vast majority of people, this extra time (often in the form of leisure) takes place in the evening, after sunset. It seems that this behaviour is an innate characteristic of human beings, as it is also observed in pre-industrial tropical societies that live according to a traditional way of life, without electricity and therefore without artificial lighting. These people get up at dawn, go about their daily activities, fall asleep on average 3-4 hours after sunset, and generally have a sleep duration roughly equivalent to ours (7 hours). Sunrise, therefore, seems to be the main parameter used as a reference for setting the social clock.
 
In countries located near the equator (as is the case for these pre-industrial societies), this social clock is very easy to adapt to the Earth’s day-night cycle since the sun rises and sets at relatively constant times throughout the year (within a few minutes, day and night each last 12 hours at the equator). However, for inhabitants of countries further from the equator, the slight tilt of the Earth’s axis of rotation creates enormous differences in the length of daylight hours in summer and winter. In Montreal, for example, there is approximately a 7-hour difference between the length of the day at the summer solstice (3:41 p.m.) and at the winter solstice (8:43 a.m.). This obviously creates a discrepancy between our biological clock and our social clock, especially in summer when the sun rises extremely early, well before the start of normal activities.
 
The main reason for the seasonal time change currently in effect, therefore, is to partially mitigate these differences, allowing sunrise to be better synchronized with our social clock. In spring and summer, we “borrow” an hour of sunlight that occurs very early in the morning and shift it to the evening, thereby lengthening the duration of daylight during the period of human activity. Increased exposure to natural light during the longer days of spring and summer can have a positive effect on energy levels and overall mood. Spending more time outdoors can also encourage greater physical activity; for example, increased daylight hours have been associated with a significant increase (6%) in children’s physical activity. Conversely, as the sun rises later and later in autumn, the shift in the clock allows sunrise to coincide with the social clock and the start of our activities (Figure 1).

Health effects

Although it has been in place in America for about a century, this approach is far from universally accepted: in fact, most surveys show that a significant proportion of respondents say they dislike seasonal time changes, especially the switch to daylight saving time, and a majority would prefer never to have to change their clocks. Difficulty falling asleep and/or waking up normally in the first few days (and in some cases the first two weeks) after the switch to daylight saving time is associated with an average reduction of 15-20 minutes in sleep duration, which seems to be a major irritant for some people.

This dissatisfaction with time changes has been amplified in recent years by several studies that have reported their potentially negative impacts on a number of health-related phenomena, the most studied being car accidents and cardiovascular diseases. Regarding accidents, the results are quite varied. Some studies report an increase in accidents in the week following the switch to daylight saving time, while others note no effect or even note a decrease. The most frequently cited example by opponents of daylight saving time is an analysis of nearly 733,000 fatal collisions that revealed a 6% increase in the week following the switch to daylight saving time between 1996 and 2017, particularly in the morning. According to the authors, approximately 28 fatal collisions could be avoided each year if the United States ended daylight saving time. However, it should be noted that these negative effects appear to be primarily short-term (and attributable to sleep disruption in the days following the time change); in the longer term, several studies show that daylight saving time is actually associated with a decrease in the total number of collisions.

Regarding cardiovascular disease, it is well documented that circadian rhythms exert a strong influence on heart function and that disruptions to these rhythms increase the risk of arrhythmias and myocardial infarction. In the context of daylight saving time, some studies suggest that the associated sleep disruption, as well as the resulting changes in cardiac activity (such as increased blood pressure and heart rate), could increase the risk of heart attack in the first few days following the switch to daylight saving time. For example, a Swedish study showed that between 1987 and 2006, the incidence of myocardial infarction increased by approximately 5% on the Monday, Tuesday, and Wednesday following the switch to summer time. Similar increases have been reported by other studies conducted in Michigan, Croatia, and Brazil, but no increase was noted in Germany, the Netherlands, Finland, or Canada. According to a meta-analysis of all these studies, the increased risk of heart attack is relatively modest, around 3%, and observed exclusively during the transition to daylight saving time.

Stable incidence of myocardial infarction

This slight increase in the incidence of cardiovascular events caused by the time change has, however, recently been challenged by a study of 168,870 documented cases of myocardial infarction from 1,124 different hospitals—a much larger sample size than in previous studies. During the period between 2013 and 2022, a comparison of myocardial infarction incidences during the week before daylight saving time, the week of daylight saving time, or the week after daylight saving time showed no significant difference, either in spring or fall (Figure 2).

Furthermore, no difference was observed in the risk of in-hospital death, stroke incidence, or revascularization procedures one week before or after the week of the spring or fall time changes. The only difference was the significant increase in the incidence of myocardial infarction in 2020, a period that coincided with the start of the COVID-19 pandemic, and it is therefore likely that this increase is linked to the sudden rise in mortality during that period.

As it has recently been pointed out, all previous studies on this topic involved a much smaller number of patients, often from the same geographic region, making it statistically difficult to precisely isolate a specific effect of the time change from among the myriad factors that can influence the risk of myocardial infarction. The high number of cases reported and the heterogeneity of the studied population (more than 1000 hospitals spread across the United States) give enormous weight to the results of this study and strongly suggest that the seasonal time change does not have a significant impact on the risk of heart attack.

Keep standard time or daylight saving time?

Even though the negative impacts of seasonal time changes on cardiovascular health are probably much less pronounced than previously thought, many people would still prefer to abandon this practice and remain on the same time year-round. In surveys, the majority of respondents tend to favor keeping daylight saving time permanently, a choice that undoubtedly reflects the numerous advantages of increased daylight hours in summer evenings, allowing people to make the most of the warmer months. However, the negative effects of daylight saving time in winter should not be overlooked; this practice delays sunrise until 8:30 a.m. at the solstice, meaning that the vast majority of people begin their day in darkness (Table 1).

The situation would be even worse for residents of areas west of the time zone, with sunrise occurring approximately an hour later, around 9:30 a.m. Such a late sunrise is incompatible with both our biological and social clocks, and experts unanimously agree that permanent daylight saving time is probably the worst possible solution, precisely because its winter impact exacerbates the discrepancy between sunrise and the start of activities. Before implementing the proposal to keep daylight saving time year-round, which is desired by the majority of the population, it would be wise to consider that several countries have tried this in the past (the United States (1973), Russia (2011-2014), and the United Kingdom (1968-1971), for example) and have all abandoned the practice, precisely because of the dissatisfaction caused by the late start of the day in winter.

Specialized medical associations, composed of experts in the fields of sleep and circadian rhythms, such as the American Academy of Sleep Medicine (AASM) and the Society for Research on Biological Rhythms (SRBR), agree that the only viable alternative to seasonal time changes is to remain on standard time year-round. In winter, this allows us to synchronize sunrise with the start of human activities, while in summer, the earlier sunset promotes the activation of processes involved in sleep (the secretion of melatonin, the sleep hormone) and encourages us to go to bed earlier. It should be mentioned, however, that at our northern latitudes, permanent standard time would imply an extremely early sunrise at the peak of summer, around 04:00 (with the appearance of dawn light even earlier, around 03:30), which is very much out of sync with the social clock of the vast majority of people (Table 1).

In conclusion, the main effect of seasonal time changes is to temporarily disrupt sleep routines. These disruptions are not inevitable, however, and can be greatly mitigated by what is known as “circadian adaptation”: in preparation for the transition to summer time (which inconveniences the most people), this simply involves setting your alarm clock forward by 15 minutes each week for the four weeks preceding the time change (or by 20 minutes for three weeks or 30 minutes for two weeks).

In terms of broader health effects, the study described earlier suggests that seasonal time changes have a minor, or even negligible, impact on the risk of heart attack, and that it would therefore be premature to invoke cardiovascular benefits to justify abolishing this practice. With the emphasis placed on the effects of daylight saving time, we also tend to forget that modern life is no longer governed exclusively by sunlight and that artificial lighting can significantly disrupt our biological clock and influence the risk of diseases, including cardiovascular disease. For example, the blue light emitted by our now ubiquitous screens has the shortest wavelength and highest light energy that the human eye can see and has a particularly destabilizing effect on circadian rhythms, especially during evening exposure. It is therefore highly likely that the omnipresence of artificial light has much more pronounced negative impacts on sleep quality and the resulting metabolic disturbances than seasonal time changes.

Finally, if we still wish to abolish these time changes, we must accept sacrificing the longest sunny evenings in summer, as the only viable alternative is to permanently maintain standard time.