For their study, the researchers worked with 25 healthy young adults who were neither sleep-deprived nor experiencing any pain. They first measured each participant's pain threshold after a full night's sleep and used Magnetic Resonance Imaging (MRI) to record their brain activity while gradually increasing heat levels were applied to their skin. The participants, on average, reported thermal discomfort at around 111 F (44 C).
Next, the researchers asked the participants to undergo the same procedure after a sleepless night. They discovered that when people are sleep-deprived, their pain threshold decreases. The majority of the participants reported thermal discomfort at around 107 F.
Their brain imaging results also changed after a sleepless night. Researchers observed a marked increase in activity in the somatosensory cortex and a decrease in activity in the nucleus accumbens and insular cortex, indicating that the neural mechanisms that manage physiological responses to painful stimuli were malfunctioning.
“If poor sleep intensifies our sensitivity to pain, as this study demonstrates, then sleep must be placed much closer to the center of patient care, especially in hospital wards,” Matthew Walker, a professor of neuroscience and psychology at University of California, Berkeley and senior author of study, said.
“Our findings suggest that patient care would be markedly improved, and hospital beds cleared sooner, if uninterrupted sleep were embraced as an integral component of healthcare management,” he added.
When the researchers began their study, they were working on the hypothesis that sleep deprivation would increase pain sensitivity. They assumed that neural mechanisms responsible for processing pain signals and activating natural pain relief would not function properly due to the loss of sleep.
They found that they were right on both accounts.
But what surprised them were the brain imaging results that showed reduced activity in the nucleus accumbens, a part of the brain involved in reward circuitry. One of the many functions of the nucleus accumbens is to increase dopamine levels in response to pain to trigger relief naturally.
“Sleep loss not only amplifies the pain-sensing regions in the brain, but [it] blocks the natural analgesia centers, too,” Walker said.
In addition, the researchers noticed that insufficient sleep also affected the level of activity in the insular cortex, another region of the brain that should be working in the presence of pain. The insular cortex evaluates pain signals and puts them in context to help the body prepare an appropriate response.
Adam Krause, the lead author of the study and a student of Walker's, explained that the insular cortex pain is part of a critical neural system that assesses and categorizes pain signals to allow natural painkillers inside the body to work. Its low activity while the body was experiencing pain indicates a neural glitch that is specifically induced by the lack of sleep.
When Walker and Krause further examined the relationship between sleep and pain in more common scenarios, they found that even the smallest alterations in a person's sleep and wake patterns could change their pain sensitivity.
“The results clearly show that even very subtle changes in nightly sleep — reductions that many of us think little of in terms of consequences – have a clear impact on your next-day pain burden,” Krause said.
The findings of their study not only highlight the importance of sleep; they also demonstrate how profoundly sleep affects the inner workings of the brain. This should be an important consideration for everyone every time they think about sacrificing sleep.