Numerous studies have demonstrated diurnal rhythms of pain sensitivity in humans, yet only one has shown an endogenous circadian heat pain rhythmicity using a constant routine procedure, a well-validated chronobiological technique. Establishing the circadian rhythm of pain sensitivity is an important first step for optimizing pain assessment and personalizing pain management strategies. The present study extended this work by employing a 26-hour ultrashort sleep-wake protocol, another validated and less invasive chronobiological method, to examine circadian regulation of mechanical (pressure) pain sensitivity in 23 healthy adults (18 males, 5 females; mean age = 29.7). All participants completed the 26-hour ultrashort sleep-wake protocol (2-hour wake/1-hour sleep cycle) that evenly distributes circadian masking factors (e.g., sleep, light, food intake, and physical activity). Pressure pain threshold was assessed nine times across the 26-hour period. Cosinor analysis revealed a strong circadian rhythm (R² = 0.93), indicating that 93% of variance in pressure pain threshold scores over time were accounted for by circadian rhythmicity. The acrophase (highest pressure path threshold, indicating lowest mechanical pain sensitivity) occurred at 1:44 PM, and the nadir (lowest pressure pain threshold, indicating highest mechanical pain sensitivity) at 1:44 AM. The amplitude of the circadian rhythm was 0.62, representing a large 1.24 z-score difference in mechanical pain sensitivity between the peak and nadir. These findings confirm previous work and further demonstrate a clear endogenous circadian rhythm in mechanical pain sensitivity. This rhythmicity has several important implications for pain assessment, clinical procedures, and chronotherapy in pain management. PERSPECTIVES: Using a 26-hour ultrashort sleep-wake protocol, a validated chronobiological technique, the authors identified a clear and robust circadian rhythm in mechanical pain sensitivity among healthy adults. These findings highlight the importance of considering circadian timing in pain assessment and may inform future approaches to personalized pain medicine.
