Why pain feels worse at night

One of the most basic human experiences, feeling physical pain,fluctuates in intensity throughout the day. Doctors and patients have known for a long time that many types of pain get worse at night. The majority of research attempts to link nighttime pain to sleep deprivation or disrupted sleep.

Scientists led by Claude Gronfier at the Lyon Neuroscience Research Centre in France have shed light on changing pain sensitivity, suggesting that our circadian clock shapes these shifts with a characteristic peak and trough of intensity at different times of the day.

People who can't dance have internal rhythms in their bodies. The body's internal clock determines when the biological processes rise and fall. Lance Kriegsfeld says that they exert control over almost all aspects of our behavior.

The work by Gronfier and his team revealed the influence of these rhythms on pain by showing that a short, painful heat stimulus was perceived to be most painful around 3 am and least painful at approximately 3 pm. "It's very exciting," says Nader Ghasemlou, a pain scientist at Queens University in Kingston, Canada, who wasn't involved in the research. "It is one of these studies that is answering questions that we've had for a long time." Advertisement

It's hard to prove that anything is driven by the body's internal clock because of the study design. A controlled laboratory setting is where researchers can rule out any environmental or behavioral factors that could cause a fluctuations in rhythm. It's not possible to tell what time it is because everything is kept constant. For at least 24 hours, participants must lie down in a dimly lit room. They can't sleep, leave, or stand to use the bathroom. Every hour, food is given as small snacks. Staff are not allowed to talk about anything related to the time. The participants' behavior is no longer rhythmic under the protocol. If the researchers spot a biological measure that has a 24 hour rhythm, that pattern can be traced back to the timing system.

Gronfier's team found 12 healthy young men who were willing to go through the protocol for 34 hours. The team tested their pain sensitivity by placing a device on their forearm that gradually increased in temperature until they felt pain. People stopped the device before it reached a certain temperature. The participants were tested with the device and asked to rate the level of pain on a scale.

Each person's body clock was measured before the team could look for rhythms. "morning larks," "night owls," and everyone in between are affected by the different rhythms of the day. The team collected hourly saliva samples to evaluate the rise of melatonin, a hormone released about two hours before someone's normal bed time, and then used this information to adjust everyone's rhythms against a single clock. There was a cycle of pain. After peaking between 3 am and 4 am, sensitivity fell to its lowest point about 12 hours later.