Why do we feel more pain at night?

As the song from the musical Les Miserables, based on the novel by Victor Hugo, says, “tigers come out at night, with their voices as soft as thunder.”

We’ve all been miserable at one time or another, when we tossed and turned around in bed, staring at the ceiling with excruciating pain in our backs (or head, ear, knee…). The pain was there during the day, but now it won’t let us rest, snapping at us like a wild tiger.

The question is, why do we feel pain more intensely at night? What does science have to say about it?

First of all, what is pain?

We have all been in pain at some point and many certainly are experiencing some pain right now. Therefore, pain is not an unknown phenomenon for anyone.

But when we need to define pain, the matter starts to get complicated.

After several changes over the years, the International Association for the Study of Pain (IASP) agreed in 2020 to define it as “an unpleasant sensory and emotional experience associated with or similar to that associated with a real or tissue potential.

Therefore, the current consensus is that pain is an experience of the senses, which has an unpleasant emotional component and is reminiscent of, or is related to, what one feels in the event of any physical damage.

What is she for?

We tend to think of this feeling as something negative, since, by definition, it is an unpleasant experience. But the human being is a complex, well-meshed machine that rarely has functions that are available without knowing why.

The purpose of pain is to warn us that something is wrong. It is a survival mechanism that helps keep us safe from dangers that can threaten our physical integrity.

To make an analogy, it is an alarm system that our brain has to tell us that we are at risk and need to be safe. And it is unpleasant precisely because we feel the need to avoid it.

But it is not a response to a stimulus, as was thought in Descartes’ time (for example, when I touch something burning and the pain saves me from getting burned because it makes me withdraw my hand). The modern conception understands pain as a product of our brain, which is the organ that tells us where, how much and in what way we feel pain.

It is clear that external stimuli (like the heat that we discussed earlier) send a signal to the peripheral nerves connected to the brain. And then the brain will process that signal and convert it into something else: called nociception.

But this is only part of the experience, as the concept of pain includes our cognitive and emotional interpretation of nociception.

Definitely, pain is not always directly related to the amount of painful stimuli we receive, since it can be perceived in its absence. An extreme example is the phantom limb phenomenon: there are people whose brain produces very real pain in a body part that has been amputated.

The control gate theory

Why, then, does the sensation increase at night, when we are safe in our bed? How does this help survival? The explanation has to do with our brain’s processing systems and the science of perception.

In the 1960s, psychologist Ronald Melzack and neuroscientist Patrick Wall proposed their theory of the floodgates of pain. According to the theory, there is a gate in the spinal cord that allows or not the passage of painful stimuli to the brain.

In other words, there will be certain things that will cause the floodgate to close and we will feel less pain and others will cause the floodgate to open and we will feel the pain more intensely. An example is the mechanical act of rubbing the skin when we receive a blow. The friction sensation competes with the pain and reduces its intensity.

In the silence of the night, the voices of these tigers sound louder, in the same way that we remember some uncomfortable situation that we experienced during the day and had almost forgotten.

Alone in the darkness, there is nothing to distract us and help to close the floodgate: neither images, nor sounds, nor interactions with others.

Worst time: 4 am

Since the 1960s, new theories, techniques and discoveries have been feeding the science of pain.

A study published in the journal Brain last September also points to circadian rhythms as a possible important agent in the phenomenon of nocturnal pain intensification.

Researcher Inès Daguet and her collaborators conducted a groundbreaking laboratory study that found that the time of day when pain is most intensely perceived (also experimentally, in this case) is at 4 am.

A possible explanation is lack of sleep, as its influence has also been demonstrated. But in Daguet’s model, the weight of circadian rhythms was much greater.

These changes could be related to the critical levels of hormones we have during the day, such as cortisol, which is related to the immune system and inflammation, and melatonin.

In spite of everything, one must not forget that this is an experimental study, in a laboratory environment. Participants are not in their natural environment (sleeping in their bed) and receive painful stimuli artificially (through a heat-inducing machine).

Predator threat alerts

Researchers Hadas Nahman-Averbuch and Christopher D. King published a commentary on the previous study, indicating that, from an evolutionary point of view, we are more vulnerable to predators at night when we are sleeping. So it makes sense that a lower intensity of stimuli is enough to wake us up from potential danger.

More research is definitely needed to understand why we feel more pain at night, but it seems that our brains keep trying to protect us so that tigers (in this case, for real) don’t eat us while we sleep.

* Rocío de la Vega de Carranza is a psychology researcher at the University of Malaga, Spain.

This article was originally published on the academic news site The Conversation and republished under a Creative Commons license. Read the original version here in Spanish.