Sleep arousal response to experimental thermal stimulation during sleep in human subjects free of pain and sleep problems

Although the interaction between sleep and pain is generating considerable interest (NIH Technology Assessment Panel, 1996), it is still unknown if chronic pain is the cause or effect of poor sleep. To further this understanding, subjects free of pain and sleep problems need to be studied in order to assess their response to pain during sleep, defined as a behavioral and a physiological state in which sensory processing is altered. (For example, while auditory perception remains active, other sensory inputs are facilitated, attenuated, or suppressed (Velluti, 1997)). The present study provides data on polygraphic responses to cool (24°C), warm (37°C), and heat pain (>46°C) stimuli applied to shoulder skin during different sleep stages: the lighter sleep stage 2, the deep stages 3&4, and REM sleep. Based on evidence from eight subjects, we found that nociceptive heat stimulation evokes a moderate level of cortical arousal during sleep. Specifically, in comparison to the response induced by a warm 37°C non-nociceptive control stimulation, the percentage of cortical arousal responses to heat pain stimuli (>46°C) was statistically greater in the lighter sleep stage 2 (48.3%) than in the deeper stages 3&4 (27.9%). A nocifensive behavioral-motor response was associated with only 2.5% of the 351 heat pain stimuli. Two other markers of sleep quality–sleep stage shift and awakening–were not influenced by the thermal stimuli. None of the subjects demonstrated any burns in the morning following the thermal stimulations applied during sleep. We conclude that the processing of nociceptive inputs is attenuated across sleep stages.