Circadian rhythms are essential for the proper functioning of our metabolism. If working hours and our social life are sometimes difficult to reconcile with our biological clock, we should remember how it works and try to synchronize with it.
Our lives, and the lives of all living beings, are punctuated by various biological rhythms that are fundamental to their proper functioning. What is it about? In fact, they correspond to “periodic or cyclic variation in the specific activity of an organism.” Depending on their duration they can be of three types:
• Ultradian rhythms, with a duration of less than 24 hours. For example, these are irregular sleep cycles or breathing or heart rhythms.
• Infradian rhythmsIt has a duration of more than 24 hours like a menstrual cycle.
• Circadian rhythms. True biological clocks that run for the equivalent of (or approximately) 24 hours (from the Latin for “circadian”). Average“around”, and dies, “day”). The most popular are sleep/wake or hormonal regulatory systems.
This last rhythm, in terms of day, is especially important. It is driven by an internal “clock” whose operator is in the brain, and precisely in the hypothalamus (located under our brain) for our species. It is composed of two neuron-rich suprachiasmatic nuclei (located under the optic chiasm), whose electrical activity oscillates over 24 hours, regulated by the cyclical activity of specific genes known as “clock genes” or “circadian genes”.
Our circadian clock is constantly synchronized by external regulating agents such as temperature or food intake… but above all light. Our retina detects light signals and transmits them to the brain and internal clock, which then synchronizes the metabolic activity of different tissues according to the received information – i.e. the time of day. Disruption of their circadian rhythms is a powerful regulator associated with significant sleep disturbances in people who are completely blind (and therefore cannot detect light).
Hormones that stop our day
In human physiology, we consider a day to be divided into two phases: activity (from 8 am to 9 am to 8 pm to 9 pm, and this corresponds to our working day, school, etc.), and rest (from 8 am to 9 pm from 8-9 am). They depend on the production of melatonin, the “sleep hormone”.
The secretion of melatonin is synchronized with the day-night cycle: in the summer it starts when the intensity of light decreases around 9-10 am, and reaches the peak of secretion in the middle of the night, between 3 and 4 am; It goes down until the sun rises.
Once the light returns, melatonin production stops and another hormone, cortisol, takes over. This “stress hormone” prepares the body for increased energy demand, which is necessary for proper functioning during the active phase. Its production is directly linked to the disappearance of melatonin – the mere presence of which inhibits the secretion of cortisol. It synchronizes cortisol production with the time of day.
In addition to this melatonin-cortisol duo, the active phase is accompanied by the production of other hormones:
• Ghrelin, which stimulates appetite. It is secreted in three peaks at 8 am, 1 pm and 6 pm.
• Leptin, a ghrelin antagonist. Secreted between 4pm and 2am with a peak at 7pm, it promotes cessation of food intake by inducing satiety and reducing appetite.
• Adiponectin, involved in the regulation of carbohydrate and lipid metabolism. It is produced throughout the day, starting at 10 am. After peaking between 11 AM and 12 PM, it gradually decreases till night. This hormone favors the use of energy substrates (sugar and lipids, etc.), rather than their storage, to generate the energy needed to support our activity phase. It improves insulin sensitivity and prevents fat accumulation.
• Insulin, which promotes the storage of energy substrates. Between 2pm and 6pm, adiponectin production increases while it begins to decrease. It prepares us for the night ahead.
These cyclic productions of light-related hormones are essential for body function. Other environmental factors can also affect circadian rhythms, especially food consumption, which can vary in the production of hormones.
When should we eat to stay in tune with our circadian rhythms?
Knowing your metabolism provides useful clues about your health, as we highlighted in a recent study. If we refer to the fluctuations of hormones that occur during the day, we can assume that we should start the day with breakfast around 8 am, when our activity phase begins, after the peak of cortisol. Since this hormone promotes storage in the form of adipose tissue, we should no longer eat after the insulin peak in the evening.
Additionally, after the insulin peak, the satiety hormone (leptin) peak soon follows, suggesting a signal to stop eating.
Thus, after 7 pm, when producing hormones involved in the use of energy substrates, it seems that eating in the morning until midday is more consistent, than it is more likely to produce them after 7 pm.
Another point to consider should be seasonal variations. In fact, in Europe, melatonin secretion is longer due to shorter days in winter and shorter due to longer days in summer. In theory, since we are a species dependent on physiological seasons, our diet should also adapt to these variations in our environment.
Many scientific studies show that living outside of circadian rhythms, especially eating late at night or changing your sleep patterns, increases your risk of developing cardiovascular diseases, obesity or type 2 diabetes.
Circadian Rhythm Disturbances: What Consequences for Our Health?
Modernization of lifestyle refers to the shift from traditional and rural life to urban and modern life, which is directly linked to various industrial revolutions and especially associated with the invention of artificial light by Thomas Edison in 1879. Artificial lighting has been a great upheaval to our lifestyle as it allows us to work at any time of the day or night and encourages staggered working hours.
In addition, globalization and the development of new technologies have encouraged the relocation of companies, forcing many employees to synchronize their work schedules with those of the countries in which they work. Combining all scenarios, almost 30% of employees reported working outside daylight hours (ie 9am to 5pm) and 19% of Europeans reported working at least 2 hours between 10pm and 5am.
Night work disrupts circadian rhythms, particularly by altering hormone levels. Several studies have shown that night shift workers produce less melatonin than workers on regular schedules. However, these disturbances are associated with an increased risk of developing cardiovascular diseases.
Artificial light keeps you awake later, which leads to other circadian clock-disrupting behaviors:
• Eat dinner after dark when you get home late from work (Eating dinner in English). As seen before, we should not eat after 7 pm – this is associated with poor insulin sensitivity, which increases the risk of developing obesity, type 2 diabetes and cardiovascular disease (especially if these late meals are made up of processed, fatty foods and sugar).
• Skipping breakfast, when morning is a physiologically favorable time to eat. This habit is associated with the risk of developing metabolic diseases.
• Change our sleep schedule. Most people accumulate sleep deprivation during work/school days and compensate by extending their sleep duration on rest days. This mismatch leads to a circadian clock disorder known as “social jet-lag,” where the sleep rhythm controlled by the circadian clock does not match the actual hours of sleep. But this social jet lag also promotes obesity and the risk of developing cardiovascular disease.
Finally, although hormones normally regulate food intake at the appropriate times of the day, external factors such as stress or frustration may encourage eating at inappropriate times. For example, in the evening after a day’s work, it is common to observe consumption behaviors of sweets to relax. Consuming sugar activates reward circuits and releases endorphins, which provide feelings of happiness and relaxation.
How to stay “in time” with your internal clock?
As we have seen, circadian rhythms are essential for the proper functioning of our body’s metabolism. If modern life, working hours, and our social interactions are sometimes difficult to reconcile with our biological clock, it is important to keep in mind how it works and try to live in rhythm with it as much as possible.
As our work has shown, there are several good habits to follow:
• Try to eat our meals between 8 am and 8 pm and minimize evening meals or snacks as much as possible.
• Limit use of screens in the evening and promote sleep by reading on paper to limit exposure to blue light from screens. Using a smartphone before going to sleep reduces the production of melatonin, while reading a paper book allows normal secretion of the hormone.
• To limit the risk of “social jet-lag”, try to adopt a regular sleep pattern during the week. Ideally, you should sleep 7 to 9 hours a night.
• Try to respect the rhythm of the seasons to better match the rhythm of your life with the cycles of light.
These simple behaviors can improve health markers and help fight certain metabolic diseases. Either way, the importance of aligning lifestyle habits with circadian rhythms is clear and beneficial…