Pain, 25 (1986) 403-410 Elsevier 403 PAI 00897 Circadian Variations of Human Flexion Reflex Giorgio Sandrini ‘, Enrico Alfonsi, Giorgio Bono, Fabio Facchinetti *, Lorenza Montalbetti and Giuseppe Nappi Headache Centre, Neurological Department, “C. Mondino” Foundation, University of Pavia, 27100 Pooia, and * Department of Obstetrics and Gynecology, Universrty of Modem, Modena (Italy) (Received 15 July 1985, revised received 20 November 1985. accepted 26 November 1985) zyxwvutsrqponmlkji Summary We investigated 8 healthy male volunteers, evaluating RI1 and RIII thresholds every 6 h starting from noon, for a 24-h period. Both reflex responses exhibited a circadian rhythmicity: the lowest values were found in the early morning (9.1 it 3.0 and 13.1 rt 4.4 rn4, respectively), while the highest values were observed at midnight (13.1 + 3.5 and 18.5 f 5.3 mA). Also mean cosinor analysis indicated the existence of a significant rhythm with acrophase at 20:12 for RI1 and 22:29 for RIII. In 4 subjects, ~-endo~~n plasma (p-EP) level was tested during the day. No correlation was observed between circadian changes of P-EP and RI11 threshold. Other factors are likely to be involved in the circadian variation of nociceptive flexion reflex in man. Introduction There is no agreement in the literature about circadian changes of pain threshold values in man. Some authors did not find any variations in this connexion [10,14,15]. Others, evaluating subjective response to different kinds of painful stimuli, described higher values in the afternoon and evening than in the morning [4,X,12]. In contrast, Procacci et al. [17], inducing pain by cutaneous thermal stimulation, observed the acrophase of the circadian cycle in the early morning. Stimulation of sural nerve elicits two different reflex responses in the biceps femoris muscle: the first (RII) is of short latency, low threshold and corresponds to a tactile reflex; the second (RIII) is of longer latency and higher threshold and ’ Correspondence to: Dr. Giorgio Sand&i, Clinica Neurologica, Via Palestro 3, 27100 Pavia, Italy. 0304-3959/86/$03.50 0 1986 Elsevier Science Publishers B.V. (Biomedical Division)