According to Public Health France, with four heatwave episodes having touched 80% of the French population, the summer of 2025 was the third hottest summer since 1900 in France. In view of the climate changes observed in recent years, it is important to realize that these episodes of intense heat will no longer be exceptional in the years to come, but will become part of our daily lives.
These significant temperature variations observed during the summer induce a extra effort for our body, especially the heart, which can lead to serious complications particularly cardiovascular.
Summer can disrupt our biological clock
But the effects of the summer period don't stop there. Indeed, with the Time change, the longer days or the heatSummer can be synonymous with disruption for our asleep, and more generally for our rcircadian rhythmthat is to say our internal biological clockIt corresponds to a cycle of approximately 24 hours and regulates many bodily functions, such as the cardiac frequency, the temperature regulation, la hormone production or againimmunityThis clock is synchronized by several time sources, including the light and the food intake.
In summer, several factors can disrupt this circadian rhythm. Although the days are longer, the Time change delays exposure to natural light in the morning, a crucial signal for synchronize the biological clock, while extending exposure to light in the evening. Furthermore, the high temperatureswhich delay or fragment sleep, as well as summer lifestyle changes Related to holidays (staying up later, more frequent socializing), can also contribute to this desynchronization. elderly and the patients with chronic diseases are particularly vulnerable due to an already weakened biological clock.
What are the effects of circadian rhythm disruption on health?
When repeated or chronic, this desynchronization, called circadian dissonance, can promote metabolic and inflammatory imbalancesand in the longer term, for example, may contribute to increasing the risk of certain pathologies such as cardiovascular diseases.
To protect oneself from this circadian desynchronization during the summer period, several tips can be applied:
- Maintain fixed and regular hours for eating
- Use a ventilateur to sleep
- Exposing oneself to light in the morning
The circadian rhythm at the heart of research at the Pasteur Institute of Lille
While the effects of heat on our biological clock are generating increasing interest, the role of the circadian rhythm in our health is already at the heart of several research projects conducted at the Pasteur Institute in Lille. The team at Dr. Hélène Duez within theINSERM UMR 1011 Nuclear Receptors, Metabolic and Cardiovascular Diseases (RNMCD), is interested in the internal biological clock and seeks to understand how it regulates metabolism and inflammation in different pathophysiological contexts such as diabetes, steatotic liver disease associated with metabolic dysfunction (MASLD), atherosclerosis and muscle regeneration.
This article was produced with the contribution of Dr. Benoît Pourcet, lecturer in team 5 “Nuclear receptors in circadian biology" from INSERM UMR 1011 RNMCD at the Pasteur Institute of Lille.
FAQ – Extreme heat and the biological clock
The biological clock corresponds to internal rhythms of approximately 24 hours which regulate many bodily functions: the sleep-wake cycle, body temperature, heart rate, hormone production, metabolism, and immunity. These rhythms are produced by our body but are synchronized by external signals, called "time cues," such as light or food intake.
The main biological clock is located in the brain, at the level of the suprachiasmatic nucleus, a small structure located in the hypothalamus. It acts like a conductor by receiving information related to light and synchronizing the various clocks present in our organs and cells.
La light is the primary synchronizer of our biological clock. food intake also plays a major role.
The light captured by our eyes transmits a signal to the brain to synchronize our biological clock. Exposure to natural light in the morning is essential for regulating our daily rhythm. Conversely, prolonged exposure to light in the evening, via screens and blue light for example, can delay falling asleep by disrupting the signals that promote sleep.
The switch to summer time causes a discrepancy between our internal biological rhythm and the social timeIt notably reduces exposure to light in the morning, which is essential for synchronizing our internal clock. Even though the body usually adapts within a few days, this change can disrupt temporarily affects sleep, alertness and certain physiological parameters, particularly in vulnerable individuals.
From a biological point of view, winter time Daylight saving time is considered closer to our body's natural rhythm, as it allows for better exposure to light in the morning. This morning light is essential for resynchronizing our biological clock. Daylight saving time allows us to enjoy more light in the evening, but can accentuate the discrepancy between our internal rhythm and our social schedules.
No. One circadian rhythm disorder This corresponds to a discrepancy between our internal biological clock and the signals from our environment (light, meal times, activity, etc.).insomnia This refers to difficulties getting enough sleep or restorative sleep. A person can therefore have a disrupted circadian rhythm without necessarily suffering from insomnia.
During a period of intense heat, the body must provide a extra effort to maintain its temperatureThis can lead to an increased heart rate, dehydration, and blood thickening, which can have serious consequences for the body, including the the heartHeat also disrupts falling asleep and sleep quality, which can indirectly contribute to a desynchronization of the biological clock.
The elderly and people with chronic diseases are particularly vulnerable to extreme heat. With age, the biological clock becomes more fragile, and the body adapts less easily to changes in rhythm. Patients with conditions such as diabetes, obesity, or hypertension may also be more susceptible to heat-related disturbances.
To preserve or resynchronize your biological clock, it is advisable to expose yourself to natural light in the morning and to keep some regular hours for meals. In summer, it is important to avoid significant changes in routine related to holidays, such as very late meals or frequent changes in sleep schedules.
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