Sleep Interruption Adds to Chronic and Spontaneous Pain

BALTIMORE, April 2 -- Sleep disturbance may set up a vicious cycle of spontaneous and chronic pain, which in turn can contribute to sleep loss, investigators here have found.

In a seven-night sleep study, women who were forced awake every hour of the night lost pain inhibition and had significantly more spontaneous pain than those whose sleep schedules were shortened or those who got a full eight hours, found Michael T. Smith, Ph.D., of John Hopkins, and colleagues.

An impaired mechanism for inhibition of pain is thought to contribute to the pathophysiology of chronic pain, the investigators reported in the April issue of Sleep. Women were chosen for the study primarily because of a gender-specific association with pain sensitivity and to make the participants a homogenous group.

People who suffer from night-time awakenings and those who have fragmented sleep -- such as firefighters, on-call medical personnel, or parents of infants -- have alterations in the natural systems that regulate pain and keep it in check, leading to the potential for spontaneous painful symptoms, Dr. Smith and colleagues noted.

Interrupted sleep appears to adversely affect central pain modulation, with a commensurate increase in spontaneous pain, they said.

"Pain not only disrupts sleep but disrupted sleep amplifies pain," Dr. Smith noted. "There is evidence emerging that insomnia predicts the development of widespread pain in individuals with regional pain. This paper provides a mechanism for how sleep fragmentation increases risk and/or exacerbates chronic pain."

The investigators chose to conduct their initial studies only in young healthy women because they wanted a homogenous sample to reduce the possibility of error of variance, and because women are consistently associated in studies with both pain sensitivity and higher rates of chronic pain.

The investigators enrolled 32 women into a seven-night sleep study in which they were monitored by polysomnography. The women slept undisturbed for eight hours the first two nights to establish baseline values, and were then randomized into three groups.

Ten patients were assigned to a forced-awakening group, in which participants were awakened once each hour for eight hours on nights three, four and five. Another 10 women were assigned to a restricted sleep opportunity group, who lost the same total sleep time as the forced awakening group by having a delayed bedtime, and 12 were assigned to be controls who were allowed to sleep undisturbed.

On night six, participants in both of the sleep-disruption groups underwent 36 hours of total sleep deprivation, and then were allowed to catch up, sleeping for 11 hours without interruption on the seventh night.

The participants completed twice-daily psychophysical assessments of mechanical pain thresholds and pain inhibition, measured as a function of diffuse noxious inhibitory controls (a noninvasive test of endogenous pain-inhibitory systems, in which noxious stimuli are applied to distant anatomic regions).

Outcome measures included laboratory pain assessments, pressure pain threshold, diffuse noxious inhibitory controls, the Pennebaker Inventory of Limbic Languidness (which measures the frequency of occurrence of a group of common physical symptoms and sensations), and Stanford Sleepiness Scale.

The authors found that participants in both the forced-awakening and restricted-sleep groups, who had 50% reductions in total sleep time, had increases in non-painful somatic symptoms during partial sleep deprivation.

"While sleep deprivation had no effect on pain thresholds, during partial sleep deprivation the forced awakening group demonstrated a significant loss of pain inhibition and an increase in spontaneous pain; neither of the other two groups showed changes in pain inhibition or spontaneous pain during partial sleep deprivation," the authors wrote.

The forced-awakening group had "robust" decrements in pain-inhibitory function as measured by a loss of diffuse noxious inhibitory controls. This group also had an increase in spontaneous painful symptoms that returned to baseline levels after the 11-hour recovery sleep.

"Somewhat surprisingly, although both partial sleep deprivation groups showed comparable increases in daytime sleepiness and spontaneous somatic symptoms after 36 hours of total sleep deprivation, neither group showed abnormal diffuse noxious inhibitory controls or pressure pain thresholds following total sleep deprivation," they wrote. "The forced-awakening group did, however, trend toward continued reductions in diffuse noxious inhibitory controls during total sleep deprivation, which might have proven to be significant with a larger sample size."

The authors noted that the study was limited by the unblinded design, and by the use of only young, healthy women, suggesting the need for further validation of their findings in men and in older adults.