Experimental pain perception remains equally active over all sleep stages.

The literature on sensory perception during sleep suggests that light sleep (Stage 2) is more responsive to external sensory stimulation (e.g. sound, electrical shock) than deep sleep (Stages 3 and 4) and REM sleep. The main objective of this study was to characterize the specificity of nociceptive stimulation to trigger sleep arousal-awakening over all sleep stages. Thirteen healthy adults (e.g. without pain or sleep problems; six female and seven male of a mean age of 24.2+/-1.3 years) were included in the study. The responses to noxious intramuscular 5% hypertonic infusion were compared to innocuous vibrotactile and to respective control stimulations: isotonic infusion and auditory stimulations. These stimulations were applied during wakefulness and were repeated during sleep. Polygraphic signals (e.g. brain activity, heart rate) signals were recorded to score sleep arousal over all sleep stages. A subjective assessment of sleep quality was made on next morning. No overnight sensitization or habituation occurred with any of the experimental stimulations. The vibratory-auditory stimulations and the noxious hypertonic infusions triggered significantly (P < 0.05) more awakenings in sleep Stage 2 and in REM than their respective control stimulations. In sleep Stage 2, both vibratory + auditory stimulations and the noxious hypertonic infusions has the same awakening response frequency (approximately 30%), however, with the noxious infusions the response frequency were similar in sleep Stages 3 and 4 (P < 0.05) and in REM (trend). Compared to the baseline night, sleep quality was lower following the night with noxious stimulation (90.1+/-2.7 and 73.3+/-7.4 mm, respectively; P < 0.03. These data suggest that pain during sleep could trigger a sleep awaking response over all sleep stages and not only in light sleep.