How can lack of sleep increase pain?

Poor-quality sleep also leads to lower levels of serotonin in the body. Serotonin is a naturally occurring neurotransmitter (brain chemical) that is associated with a calming, anxiety-reducing feeling in the body. When serotonin is depleted from lack of sleep, the result is an increase in sensitivity to pain, as well as increased feelings of anxiety, malaise, and even depression.

Poor sleep lowers your pain threshold, both making you more sensitive to pain and making your pain more intense. It's easy to see how not sleeping and feeling a bit crabby can make you tolerate your pain less well. But there's more to it than that. Sleep seems to directly affect how the brain experiences sensations as painful or not.

In an interesting experiment with healthy adults who didn't have pain problems, researchers from the Henry Ford Health System measured how hot something had to be before heat was felt as pain. During the experiment, some people were allowed to spend a full 8 hours in bed at night -- others were only allowed 4 hours. Their sleep stages were carefully monitored to see who might be experiencing disrupted sleep because of abnormal sleep cycles. The pain threshold measured the next day dropped by one-fourth when people had only been allowed to sleep 4 hours the night before. In addition, the pain threshold dropped by one-third when people had disrupted sleep, with an abnormal cycling of non-rapid eye movement (REM) and REM sleep. These experiments clearly show that not sleeping enough or having poor-quality sleep makes you more sensitive to pain.

The relationship between sleep and pain is one we don't yet fully understand. What we do know -- and what anyone who suffers from chronic pain can attest to -- is that each can have a significant effect on the other. The presence of pain can make it hard to fall asleep and stay asleep, and being sleep deprived can make a person feel more sensitive to aches and pains. Studies that have examined the relationship between sleep and pain found these links to be true -- and also found that it doesn't take a lot of lost or disrupted sleep to have an effect on how we experience pain:

• One study showed 12 women who were deprived of sleep for only 3 consecutive nights experienced increased sensitivity to pain. Pain sensitivity began to increase after a single night of disrupted sleep and grew worse each night.
• Another study tested healthy men for pain sensitivity after a short period of total sleep deprivation. Researchers found that total sleep deprivation dramatically decreased the men's threshold for pain. A recovery sleep period after sleep deprivation restored the men's pain threshold.
• Recent research into fibromyalgia showed that women who were sleep deprived had as much as 5 times higher a risk of developing the chronic pain syndrome, over a 10-year period, as women who were not sleep deprived.
• A sleep-pain study performed on rats showed that when deprived of REM sleep, the rats experienced a significant increase in sensitivity to several types of pain stimuli.