Diurnal variations in pain perception and thermal sensitivity.

Pain and thermal sensitivity thresholds in healthy volunteers were examined for diurnal variations. The subjects were 11 men aged between 22 and 27 years (means = 23.5, S.D. = 1.5). Data were collected for 2 days, with 7 measurements per day. To ensure the pain specificity of the results the subliminal modality, i.e., thermal sensitivity thresholds to warm and cold stimuli, was investigated in addition to the threshold for perception of heat pain. Assessments were made on the right hand and foot, the stimuli being presented with a thermoelectric contact-thermode. Despite the influence of variables other than time of day (45-56% of the total variance), diurnal variations were found for some subjects on the pain threshold measure (significant correlation between days and relatively high frequency of 24 h component in Fourier analysis spectra). However, they could not be demonstrated for the thermal sensitivity measures. The diurnal variations in pain perception thresholds did not have a consistent pattern over all subjects (Friedman test). The small diurnal variations with interindividual differences in the pattern are therefore not sufficient to explain the variations seen in clinical pain, but they may be useful in detecting pain modulators by investigating correlations.