Circadian variations of human flexion reflex

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Abstract
We investigated 8 healthy male volunteers, evaluating RII 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 .+-. 3.0 and 13.1 .+-. 4.4 mA, respectively), while the highest values were observed at midnight (13.1 .+-. 3.5 and 18.5 .+-. 5.3 mA). Also mean cosinor analysis indicated the existence of a significant rhythm with acrophase at 20:12 for RII and 22:29 for RIII. In 4 subjects, .beta.-endorphin plasma (.beta.-EP) level was tested during the day. No correlation was observed between circadian changes of .beta.-EP and RIII threshold. Other factors are likely to be involved in the circadian variation of nociceptive flexion reflex in man.