Why does the pain seem stronger at night? – Focus.it

Have you ever noticed that when something hurts, the pain feels stronger at night? Doctors and scientists have known this for a long time: the perception of pain is not always the same, but it seems to fluctuate throughout the day. Past research has tried to link the phenomenon to sleep deprivation, but without great results. Now a study published in the journal Brain proposes another explanation: pain, as well as appetite or sleep, would also seem to follow circadian rhythms.

ups and downs. Circadian rhythms are cycles lasting about 24 hours in which some physiological processes are repeated regularly. As we know, they are regulated both by a kind of internal ticking present in all living organisms (the biological clock), and by external factors such as light or temperature.

A group of neuroscientists from the Neuroscience Research Center in Lyon, France, demonstrated their influence on the perception of a painful stimulus: a short, too hot pulse was perceived as most painful at three in the morning, and minimally painful at three in the morning. afternoon. In between these two extremes, the perception of pain increased from the afternoon to the early hours of the morning, and decreased from the early hours of the morning to the following afternoon.

Tortured for science. To isolate the influence of circadian rhythms from other possible stimuli, the 12 participants in the study were subjected to a rigid protocol called “constant routine”: they remained in a semi-lying position but without sleep for 34 hours, with poor lighting. and identical snacks every two hours. In these conditions there is no signal that refers to the time of day: therefore, if the researchers notice a biological phenomenon that follows a rhythm calibrated over 24 hours, it means that that trend is regulated. by an internal mechanism and which is part of the circadian rhythms.

How bad does it hurt? Every two hours, the scientists tested the pain sensitivity of the unfortunate by placing a device on their arms that gradually increased in temperature, one degree at a time. Typically, participants asked to stop it when it reached 46 degrees Celsius. In another battery of tests, the volunteers experienced the painful stimulus at specific temperatures (42, 44 and 46 degrees), and explained to the researchers how much pain they felt, as if they had to visually represent it on a scale.

Finally, the results were calibrated on individual characteristics – for example, the aptitude for waking up early or going to bed late was taken into account, of “owls” and “larks”and it was all synchronized into a single large clock to compare the data.

Sleep has nothing to do with it. At this point, a very clear cycle emerged. Pain sensitivity peaked between three and four in the morning before hitting its lowest point 12 hours later. These constant rhythms work only for painful stimuli (for example, boiling heat) and not for the perception of heat and cold (for example, tepid heat).

Since the participants hadn’t slept, the team also had to rule out that the accumulation of sleep influenced the perceived pain. If this were the case, the sensitivity of the volunteers should have increased as the urge to sleep grew, rather than having a rhythmic, increasing and then decreasing trend. So even this aspect was discarded: according to the authors of the study, 80% of the data collected can be explained by circadian rhythms.

The rhythm of the cells. What causes such constant and precise variations? One hypothesis is that it depends on the molecular clock present in every cell of our body, finely synchronized with the brain: the perceived pain could depend on the rhythm with which those cells identify the pain.

Targeted interventions. Research may help give pain medications more appropriately: Patients in hospitals have been known to complain of more pain at night, yet painkillers are usually not given until the next morning. Another important aspect will be to study if, and how, the fluctuations discovered change between men and women, between young and old and between people of different ethnic groups. And how they are in people suffering from chronic diseases: it is known, for example, that those suffering from inflammatory diseases such as arthritis experience more pain in the morning.

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Why does the pain seem stronger at night? – Focus.it


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