Insomnia may lead to lower pain tolerance

People with insomnia and other sleep problems have increased sensitivity to pain, according to a new study. The effect on pain tolerance appears strongest in people who suffer from both insomnia and chronic pain, who may benefit from treatments targeting both conditions.

The study included more than 10,400 adults from a large, ongoing Norwegian health study. Each subject underwent a standard test of pain sensitivity - the cold pressor test - in which subjects are asked to keep their hand submerged in a cold water bath.

Subjects were asked about various types of sleep impairment, including insomnia, total sleep time, and sleep latency (time to falling asleep), and researchers assessed the relationships between measures of sleep impairment and pain sensitivity.

The study also looked at other factors potentially affecting sleep impairment and pain perception, including chronic (persistent or recurring) pain and psychological distress (such as depression and anxiety).

Overall, 32 per cent of participants were able to keep their hand in the cold water throughout the 106-second test.

Participants with insomnia were more likely to take their hand out early: 42 per cent did so, compared with 31 per cent of those without insomnia.

Pain sensitivity increased with both the frequency and severity of insomnia. For example, compared with individuals who reported no insomnia, rates of reduced pain tolerance were 52 per cent higher for subjects reporting insomnia more than once weekly versus 24 per cent for those with insomnia once monthly.

Pain sensitivity was also linked to sleep latency, although not to total sleep time. The relationships were unchanged after adjustment for age and sex. The effect was smaller, but still significant, after further adjustment for psychological distress.

There was also strong joint (synergistic) effect of insomnia and chronic pain on pain tolerance. Patients reporting high problems with both insomnia and chronic pain were more than twice as likely to have reduced tolerance to pain.

Many patients who experience sleep impairment and pain face high costs and personal difficulties. "While there is clearly a strong relationship between pain and sleep, such that insomnia increases both the likelihood and severity of clinical pain," said Borge Sivertsen, of the Norwegian Institute of Public Health. "It is not clear exactly why this is the case," he said.