Sunday March 25, we switch to summer time. This is an opportunity to return to the impact of changing the sleep-wake rhythm on our health. The importance of circadian rhythms recently won the 2017 Nobel Prize in Physiology for researchers who uncovered how they work. Decisive for well-being and the prevention of obesity and type II diabetes, respecting our internal rhythms is a major key to preserving our health.
Not only is the sun essential to life on Earth, but it also controls the pace at which that life unfolds. All living beings, whether bacteria, plants or animals, have evolved to coordinate their activities with the day-night cycle associated with the Earth’s rotation. These “circadian rhythms” (from the Latin circa, around and diem, day) make it possible to synchronize physiological functions with a specific moment of the day: the flowering of plants, the migration of birds and butterflies, or even certain physiological functions such as secretion of various hormones are all examples of phenomena that depend on circadian rhythms specific to each living species. This biological clock that exists inside each cell therefore represents a fundamental mechanism of life on Earth, preciously preserved during the billions of years of its evolution.
In more complex animals like humans, this biological clock is dictated by a group of nerve cells located in the hypothalamus of the brain. These cells are particularly sensitive to the intensity of light captured by the retina of the eye and are able to integrate this information to control the sleep-wake cycle as well as several aspects of metabolism. This is a phenomenon of great importance, because the majority of our genes show fluctuations according to the time of day, which makes circadian rhythms one of the most important mechanisms for controlling the functioning of the human body.
Changing rhythms harms health
The importance of this biological clock is well illustrated by the discomfort felt when it is disturbed, for example following an air journey through several time zones. The sudden change in brightness causes physiological functions that normally take place at a specific time of day to become out of step with the current time, disrupting normal physiology.
Several studies indicate that our current lifestyle can also disrupt the biological clock and lead to long-term adverse health effects. During evolution, our species has adapted to eating and sleeping according to the rhythm dictated by the day-night rhythm: since Homo sapiens was born in Africa, near the equator, this therefore means that our metabolism has evolved to function optimally for roughly equivalent lengths of day and night (12 hours each).
LED bulbs cause light overexposure
The advent of electric lighting has completely changed these day-night cycles and thereby human behavior, for example by favoring the consumption of food over longer periods and by reducing the duration of sleep. This excessive nocturnal lighting, which is sometimes called light pollution, has several unfortunate consequences on the fauna and flora (artificial light disrupts pollination, for example) and the situation is likely to deteriorate with the generalization of lighting systems even more efficient such as light-emitting diode (LED) lamps which cause over-illumination.
Disturbed sleep cycle: obesity and diabetes favored
In humans, one of the best documented effects of disruption of the day-night cycle is its impact on blood sugar. During the night, when no food is consumed, the liver makes glucose and secretes it into the circulation, while during the day this system is silenced due to the ample amounts of sugar from food. Thanks to a sophisticated system of regulatory genes which are expressed according to a very elegant choreography, the biological clock allows the body to control blood sugar levels completely independently. On the other hand, when the sleep-wake cycle is disturbed, this system becomes inefficient and the resulting fluctuations in blood sugar can over time affect insulin production and lead to type 2 diabetes. These observations also explain why a very large number of studies have shown that people who have abnormal sleep-wake cycles, night workers for example, are at higher risk of obesity and diabetes.
Regulate the sleep-wake cycle and eat little in the evening
Over the past few years, several studies have indicated that a simple way to avoid these metabolic disturbances is to concentrate food intake over a short period of the day. This new concept, called “time-restricted diet” consists of limiting food consumption over a specific period of the day and especially avoiding eating during the evening. In practice, this means reconnecting with the way humans have eaten during evolution, i.e. alternating periods of food intake with periods of fasting of at least 12 hours, which usually coincide with sleep. The work carried out to date indicates that this type of diet prevents or delays the progression of several diseases, including atherosclerosis, diabetes, certain cancers, and neurodegenerative diseases, possibly by regulating circadian rhythms and optimizing metabolic efficiency. The elucidation of the functioning of circadian rhythms therefore not only represents a major advance in the understanding of a mechanism absolutely essential to life, but also opens the way to new perspectives for the treatment of several chronic diseases caused by the disruption of these circadian rhythms.
Panda S. Circadian physiology of metabolism. Science; 354: 1008-1015.