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Inserm researchers have discovered that the intensity of pain is controlled by our internal clock. It oscillates over 24 hours with a peak at night and a drop in the afternoon.
The sleep/wake system, body temperature, blood pressure, hormone production or even memory: many body functions are regulated by an internal clock set to a rhythm of around 24 hours, which we called circadian rhythm. But after following 12 young adults in their laboratory placed in conditions of temporal isolation and constant routine, the Inserm researchers discovered that pain was also controlled by our internal clock.
For this experiment, the researchers exposed the participants’ forearms to a heat source every 2 hours. “On the one hand the participants had to indicate when the stimulus became painful during the increase in temperature, and on the other hand they had to rate the intensity of the pain on a scale of 1 to 10 during the application. at a temperature of 42, 44 or 46 degrees Celsius. Two complementary approaches intended to check the consistency of the results” they explain.
The peak of pain felt between 3 and 4 a.m.
The researchers observed in all the subjects a rhythmicity of the painful sensation, during the 24 hours. The intensity of the pain follows a sinusoidal curve over 24 hours: it reaches its maximum intensity between 3 and 4 am and it is at its lowest around 3 and 4 pm in the afternoon.
“It is not known why sensitivity is greatest in the middle of the night. It may be thought 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 environment and more easily prone to injury; this warning signal could therefore be less necessary” underlines Claude Gronfier of the Lyon Neuroscience Research Center, main author of the study.
More intense pain in case of lack of sleep
In addition, the researchers found that pain sensitivity increased with sleep debt: the more sleep deprived, the more intense the pain. “But the internal clock is responsible for 80% of the variation in pain sensation over 24 hours, compared to only 20% for sleep,” they explain.
This discovery is part of the concept of personalized medicine: adapting the analgesic treatment of patients suffering from chronic pain, taking into account the biological rhythm of each, could indeed increase its effectiveness while reducing the necessary dose and the potential adverse effects.
Source : Circadian rhythmicity of pain sensitivity in humansBrain, July 2022
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