Winter Time: What the Time Change Does to Your Body and What to Do, According to Studies

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The shift to standard time occurs on the last Sunday of October : at 3 a.m., it becomes 2 a.m. again. Researchers refer to a return tostandard time, the time that most closely approximates solar time; daylight saving time, in spring, advances clocks by one hour relative to it.

The next day, an alarm ringing at 7 a.m. corresponds to 8 a.m. old time: mornings are brighter, but night falls an hour earlier upon returning from work. Since days are already getting shorter, it is difficult to separate the effect of the time change from that of the season: some of the increases attributed to the time change disappear when compared with regions that do not change their clocks.

Thecircadian clock organizes hunger, sleep, and alertness over approximately 24 hours, and resets each day thanks to light. According to a position statement by chronobiologists, daylight is generally 100 to 1,000 times more intense than indoor lighting, making it far more effective for resetting the clock.

<<<26>>> Melatonin mélatonine is the hormone that signals nighttime. Its secretion begins in the evening in dim light and stops in bright light, and household lighting is enough to disrupt it: in 116 volunteers aged 18 to 30, indoor lighting below 200 lux in the 8 hours before bedtime, compared to dim light below 3 lux, delayed the onset of secretion in 99% of them and shortened its duration by approximately 90 minutes.

Bright evening light shifts the clock later: in a laboratory trial, reading on a backlit e-reader before sleep, rather than a printed book, delayed sleep onset and the clock, reduced melatonin, and decreased alertness the next morning.

In spring, one must wake an hour earlier for the biological clock, in the dim morning light that returns: this is a net loss of sleep and, over 8 weeks of follow-up published in Current Biology, activity time did not adjust to it, especially in "evening people." In autumn, the alarm falls an hour later for the internal clock, and the same follow-up shows easy adjustment.

A British learned society on sleep concludes, in a position statement published in 2025, that circadian health and sleep are negatively affected by imposed time changes, especially forward shifts, thus in spring, and positively affected by natural morning light. A systematic review of 27 studies points in the same direction, with heterogeneous results: shorter and poorer sleep, increased drowsiness, especially in spring and in "evening people."

Autumn is not neutral either. A reference review emphasizes that the hour gained is hardly recovered in sleep on the night of the change, and that five consecutive days of earlier clock-based wake times can result in a net loss of sleep over the week.

This study compares different people from one Sunday to another, a limitation acknowledged by its authors. In a Canadian cohort of 30,097 people aged 45 to 85, those interviewed the week following the standard time change reported more often being dissatisfied with their sleep than those interviewed the previous week (28% versus 22.6%). They reported more difficulty falling asleep (7.1% versus 3.3%), more bothersome nighttime awakenings (12.9% versus 8.2%), and more daytime sleepiness (7.3% versus 3.6%). In spring, only a 9-minute reduction in sleep duration appeared. The authors describe a transient increase in sleep disorders after the switch to standard time.

How long does adaptation take? No study provides a universal figure: the autumn adjustment is described as easy, and the increase in complaints occurs in the first week. As an estimate, not a study result, count on anywhere from a few days to one week.

Myocardial infarction. A 2019 meta-analysis (7 studies, over 115,000 people) found a modest but significant increase in risk after the spring daylight saving time change (odds ratio 1.05) and risk comparable to control periods the week following the autumn standard time change (1.01, not significant). The 2024 meta-analysis (12 studies, 10 countries) confirms: relative risk of 1.04 in spring, 1.02 in autumn, not significant, reduced to 1.00 after excluding one inconsistent study. However, an Andalusian registry noted an observed-to-expected ratio of 1.06 for myocardial infarction around the autumn time change, just barely significant: an isolated regional signal.

Stroke. In Finland, from 2004 to 2013, hospitalizations for ischemic stroke increased by 8% during the first two days following time changes, analyzed together, but not over the entire week; the increase reached 20% in those over 65 years old.

Road. Among 732,835 fatal accidents recorded in the United States from 1996 to 2017, the spring daylight saving time change was associated with a 6% increase in risk, especially in the morning, with no effect at the return to standard time. A 2026 analysis, which compares with two states that do not change their clocks, no longer finds an overall effect, but an increase of approximately 26% in morning fatal accidents after the spring change, and suggests that the apparent autumn effect is mainly due to the season itself.

Its authors work for a road safety research institute linked to the insurance sector. A review of 157 studies conducted in 36 countries associates the switch to standard time, each time based on a single high-quality study, with a decrease in all-cause mortality and workplace accidents, but also with more physical assaults.

These are fairly severe depressions leading to hospitalization. The authors propose a hypothesis: the sudden advance of sunset, which opens a long period of short days, may be experienced as a shock. The review of 157 studies found no clear and consistent effect on mental health.

A good reason to pay attention to your mood in November. Our guide on mental fatigue helps distinguish temporary fatigue from a condition warranting medical consultation.

"Evening" or "morning" types. Chronotype refers to the time at which your internal clock naturally places sleep. "Evening" types are most affected in spring, as shown by Current Biology monitoring and a review of 27 studies. In autumn, this is an untested hypothesis in these studies: setting the clock back could bring them closer to their natural rhythm, and very "morning" types risk waking up early on the first few days.

Adolescents. Naturally more "evening" types, they showed increased daytime drowsiness after the transition to daylight saving time, up to the third week, more so in older and more "evening" adolescents. We found no equivalent data for winter time.

Babies and young children. We identified no studies on infants. In practice, adjust bedtime, meals, and naps by 10 to 15 minutes per day in the week before or after, and expose them to daylight in the morning: this is practical advice, not a study finding.

Seniors. The sleep studies cited above involved adults aged 43 to 85, who reported more insomnia in the week following the transition to winter time; in the Finnish registry, combining both time changes, the increase in strokes in the first two days reached 20% after age 65. Regular schedules and outdoor time in daylight remain good practical recommendations.

<<<38>>> Seasonal depression dépression saisonnière is a recurring depression that develops in autumn or winter and disappears in spring. Its prevalence ranges from 1% to 10% depending on latitude and measurement methods; it is more common in women and younger people, and more represented in bipolar disorder. Only a healthcare professional can make the diagnosis. A review of 11 studies notes heavier dinners and increased cravings for starches in patients; our analysis of sugar and sleep details this cycle.

Light therapy consists of exposing yourself daily, with eyes open, to a very bright lamp. A 2005 meta-analysis attributed a large effect to it in seasonal depression (effect size of 0.84 across 8 trials); by convention, 0.2 corresponds to a small effect, 0.5 to a medium effect, and 0.8 to a large effect. A more recent meta-analysis (lamps of at least 1,000 lux versus light of 400 lux or less) finds a more modest but significant effect (−0.37, 18 trials, 610 patients), with studies of variable quality.

Blue light lamps have not shown efficacy against inactive light in a meta-analysis of 9 trials. Going outside in the morning does not replace treatment when depression is established.

Vitamin D is synthesized by the skin under the effect of sunlight, hence the idea of a link with mood in winter. Meta-analyses are conflicting. The one of 31 trials and 24,189 participants finds a small decrease in depressive symptoms per 1,000 IU per day (−0.32), more marked in already depressed individuals (−0.57), but only in short trials: beyond 24 weeks, the effect is no longer significant. In elderly people, 11 studies and 21,561 participants show no significant effect (−0.10). And for seasonal depression, a systematic review concludes that no nutritional intervention tested, including vitamin D, has shown benefit.

Vitamin D maintains its place in winter to cover needs when sunlight is scarce, as explained in our guides on vitamin D deficiency and on when and how to take it; based on current trial evidence, we cannot attribute an effect on seasonal low mood. Our Plant-based Vitamin D3, presented at the beginning of the article, provides 1,000 IU per drop, at a rate of 2 drops per day during a meal in adults. In case of pregnancy or breastfeeding, consult a healthcare practitioner.

Saffron has been studied in adults without diagnosed depression. In a British trial, 56 adults aged 18 to 54 with low mood or stress took 30 mg per day (two 15 mg capsules) of a standardized extract for 8 weeks: their depression scores decreased more than under placebo. In a trial from a German university, in 165 adults suffering from moderate insomnia, 20 or 30 mg of the same extract for 4 weeks reduced insomnia symptoms slightly more than placebo. In this second trial, the extract was taken in the evening, approximately one hour before bedtime. These trials involve the extract from our Saffron, dosed at 30 mg per capsule and to be taken in the morning with a meal, and count among their authors employees of its manufacturer; none addresses the time change nor seasonal depression. Our article on saffron and sleep details them. Saffron is not recommended during pregnancy, breastfeeding and before age 12, and requires medical advice when taking antidepressants, anxiolytics, or anticoagulants.

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When taken as a supplement, melatonin somewhat promotes falling asleep and can shift the clock, earlier or later depending on the time of intake.

In 34 adults aged 18 to 42, 0.5 mg of melatonin for 3 days produced maximum clock advance when taken in the afternoon, 2 to 4 hours before the natural onset of secretion, and delays when taken just after waking; the optimal time was later for 0.5 mg than for 3 mg. For falling asleep, a meta-analysis of 19 trials and 1,683 people suffering from primary sleep disorders finds modest effects: 7 minutes gained in falling asleep, 8 minutes more sleep, and a small improvement in sleep quality (0.22).

For the winter time change, the clock must reset later, whereas melatonin, taken before the natural onset of its secretion, advances it rather: it is therefore not, in itself, the tool for autumn resetting, and we found no trials on time changes. These results concern melatonin alone, at specific doses and at specific times, and do not transfer directly to other doses or other times of intake.

For melatonin in general, the 2002 review reports cases in people with epilepsy or taking warfarin, an anticoagulant: in these situations, consult a healthcare practitioner. Other options exist, such as glycine, presented in our glycine guide.

Caffeine. In a double-blind trial, the equivalent of a double espresso taken 3 hours before usual bedtime delayed the clock by approximately 40 minutes, almost half the effect of 3 hours of bright light at about 3,000 lux at bedtime. It also interferes with falling asleep: avoid coffee late in the day during the week of the time change. Our guide caffeine and sleep details its duration of action.

Screens and evening light. The evening extends under artificial light, which delays melatonin: dim lighting and screens in the hour before bedtime, as explained in our article on eye strain and screens.

Exercise. In 99 adults aged 18 to 30 and 59 to 75 years, one hour of moderate exercise on a treadmill, 3 days in a row, advanced the clock to 7 a.m. or between 1 and 4 p.m., and delayed it between 7 and 10 p.m. To support the fall clock adjustment toward later times, moderate activity in the evening, between 7 and 10 p.m. in this study, goes in the right direction, provided you don't schedule it too close to bedtime (practical tip).

Meals. In 10 young men, delaying meals by 5 hours for 6 days did not shift the central clock markers (melatonin, cortisol), but delayed blood glucose rhythm by nearly 6 hours. Gradually shifting breakfast and dinner times can thus help keep organ clocks synchronized, a practical tip derived from this data.

Alcohol and naps. We did not identify a specific study: as a practical tip, avoid compensating for a difficult evening with a drink, and prefer a short nap in early afternoon to a long late-afternoon nap. Magnesium is often associated with evening relaxation: our Magnesium+ combines microencapsulated magnesium oxide, magnesium malate, taurine and vitamin B6, at a dosage of 4 capsules per day; no trial has tested it for daylight saving time adjustment, and our article magnesium: morning or evening? helps you choose the best time to take it.

Eliminating time changes requires choosing between standard time, winter time, and permanent daylight saving time. The 2019 position of chronobiologists defends thepermanent standard time, which would eliminate the acute effects of changes such as the prolonged gap between social clock and solar clock imposed by daylight saving time. An American learned society of sleep medicine reaffirmed this in 2024, as did the British learned society mentioned above.

Learned societies agree on one point: the time changes themselves, especially in spring, strain the biological clock; the 2026 review invites consideration of their beneficial effects as well. In the meantime, winter time remains the easier of the two to navigate.

This protocol, practical advice not tested as such, assembles the levers measured in studies to gently reset the clock.

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