According to French researchers, the circadian rhythm, this internal biological clock set over a 24-hour day, would have a major role in our perception of pain.
- Almost all of the body’s functions are subject to the 24-hour circadian rhythm, and this clock is synchronized primarily through natural light. Disorders of this rhythm can have consequences on sleep as well as on metabolism, the functioning of the cardiovascular system, the immune system, etc.
- The study in question reveals that we would be most sensitive to pain in the middle of the night, between 3 and 4 am, and the least sensitive in the middle of the afternoon, between 3 and 4 pm.
We have known for a long time that sufferers feel pain more strongly at night than during the day, but why? This is the question asked by a team of neuroscientists from the National Institute of Health and Medical Research (Inserm) at the University of Lyon, and they say they have put their finger on a semblance of an answer: pain intensity is regulated by the internal circadian clock.
As a reminder, the circadian rhythm is defined by the alternation between wakefulness and sleep over a cycle of approximately 24 hours. It is a kind of internal clock that regulates our biological rhythms, and on which almost all the functions of the body depend: the sleep/wake system, the cardiovascular system, the immune system, the production of hormones, the mood, memory… But also, therefore, pain.
Volunteers kept awake and pain tested
To achieve their results, published on September 9 in the journal Brain, the researchers solicited twelve young volunteers, men only (to avoid disturbances linked to the menstrual cycle), in good health and without a history of sleep disorders. They followed them during a “constant routine protocol”, a method regularly used by research to study circadian cycles.
On the program: isolated in a laboratory, the participants had to stay awake for 34 hours, always in a semi-recumbent position, with the prohibition even to get up to go to the toilet. They had no concept of time, no smartphone or natural light, no fixed meal times – a snack was given to them every hour.
In parallel, the volunteers were tested on their pain tolerance by exposing their arm to a heat source every two hours (from 42 to 46°C). They had to indicate when it became unbearable, and assess the intensity of pain on a scale of 1 to 10. The objective, to verify the hypothesis that the sensation of pain is rhythmic over a 24-hour cycle.
More sensitive to pain in the middle of the night
The results did not disappoint: the circadian clock does have an impact on pain tolerance, which fluctuates throughout the day and night. The study reveals, for example, that we would be most sensitive to pain in the middle of the night, between 3 and 4 a.m., and the least sensitive in the middle of the afternoon, between 3 and 4 p.m., i.e. half a day more late. “We do not know why the sensitivity is maximum in the middle of the night, explains the main author of the study, Claude Gronfier in a communicated. We can think that evolution has put this in place in order to be awakened quickly in the event of painful contact and to avoid a vital threat. During the day, the individual is aware of the surroundings and more easily prone to injury; this warning signal may therefore be less necessary.”
The researchers also made another discovery: the greater the sleep debt, the more intense the pain felt. “Sleep is often said to have an analgesic action, but by mathematically modeling our results, we show that the internal clock is responsible for 80% of the variation in pain sensation over 24 hours, against only 20% for the sleep.” In other words, your lack of sleep does not help our pain tolerance, of course, but it is first and foremost the time of day that affects our ability to bear it.
“It is legitimate to think that improving the synchronization of biological rhythms and/or the quality of sleep in individuals suffering from chronic pain could contribute to better therapeutic management”, concludes the Inserm researcher. “How and when do you sleep?”, this can be the question to ask if you are in pain somewhere…