Sensitivity of sleep stages to painful thermal stimuli

Many modalities of both acute and chronic pain have been shown to disrupt sleep. Any differences in the intensity of thermal noxious stimulus required to produce arousal from stage 2, slow-wave sleep (SWS) and rapid eye movement (REM) sleep is unclear. An assessment of reactions of seven male (age 22 +/- 2.9 years) and three female subjects (age 21.0 +/- 1.0 years) to a range of gradually increasing temperatures was used both when awake and asleep. When awake, subjects assigned five different descriptors to the increasing heat stimulus. During the different stages of sleep, temperatures were increased over the same range as when awake until the subjects aroused from sleep. The possible fluctuations in pain perception due to a time-of-night effect were assessed in awake subjects over a 12-h period from 19:00 to 07:00 hours. During sleep, arousals occurred at significantly higher temperatures during SWS (P < 0.01) and REM sleep (P < 0.05) than during stage 2 sleep. The temperatures causing arousals during SWS and REM sleep were not significantly different and were equivalent to temperatures causing pain tolerance when awake. No changes in pain perception due to time of night were observed. The results show that a higher intensity of thermal noxious stimulus is required to cause arousal from SWS and REM sleep when compared with stage 2 sleep. This would confirm the suspicion that REM sleep and SWS are relatively, and possibly equally, resistant to disruption by noxious stimuli.