Link found between pain sensitivity and circadian rhythm

A human’s circadian rhythm and melatonin levels. Credit: Srruhh, CC BY-SA 4.0 , via Wikimedia commons

A team of researchers with members from Université Claude Bernard Lyon, Hospices Civils de Lyon and Paris-Saclay Université has found a link between pain sensitivity and a person’s circadian rhythm. Their paper was published in the journal BrainThe group describes how they tested for pain sensitivity in male volunteers who were subject to a “constant routine protocol.”

Since long ago, medical scientists have known that pain sufferers feel more intensely at night. This phenomenon has been a mystery for many years. In this new effort, the researchers found a clue—the circadian rhythm.

The researchers sought the help of 12 healthy male volunteers who had no sleep problems to test the hypothesis that pain sensitivity could be linked to a person’s circadian rhythm. Each participant underwent a consistent routine protocol and was also tested for pain tolerance.

A constant routine protocol involves having a person lie down in a semi-recumbent position for a period of time—typically 24 hours. Any rhythm-related activities are eliminated during that time. The light is kept steady as well as the noise. Each hour, food is only given in small amounts. Participants are not allowed to go to the bathroom. This protocol is used to study circadian rhythms and how they are built into the human body.

The new effort involved 12 volunteers who participated for 34 hours. They were then tested for pain sensitivity every 2 hours. The pain tests involved slowly increasing temperature of a device taped on the body to feel pain. The researchers noted that the volunteers typically halted the device at approximately 115° F. For their study, the researchers first measured the circadian rhythm for each of the volunteers by taking saliva samples every two hours over a 24-hour period and noting melatonin levels.

Researchers found a strong indicator of the circadian rhythm’s impact on pain tolerance. They found that pain tolerance was weakest around 3 or 4 AM and strongest at 3 or 4. p.m. To verify that the sensitivity was due to pain and not thermal stimuli, researchers repeated the exercise with the pain detection test reduced to a warmth test. They found no differences between the tolerances based on the time of the day.

Researchers plan to continue their research, and to discover how the circadian rhythm controls pain tolerance.

Biomarkers help to identify the circadian rhythm in the human epidermis.

More information:
Inès Daguet et al, Circadian rhythmicity of pain sensitivity in humans, Brain (2022). DOI: 10.1093/brain/awac147

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Link between pain sensitivity & circadian rhythm (2022 – September 21)
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