understanding of the daily rhythmicity in nociception is important for the standardization of studies of analgesic drugs . yet , few studies have investigated the daily rhythmicity in nociception . although studies on rats   and golden hamsters   have indicated the occurrence of greater pain sensitivity during the dark phase of the light - dark cycle , another study on rats indicated the occurrence of greater sensitivity during the light phase , and a study on mice indicated the occurrence of two daily peaks in sensitivity , one during the light phase and one during the dark phase . they were housed in microlon cages maintained at 25  1c under an l12:d12 light - dark cycle . nociception was evaluated by eddy 's hot plate method , tail immersion method , and tail clip method . the latency between the noxious stimulus and the animal 's response was recorded as reaction time . rats previously adapted to an l12:d12 light - dark cycle were divided into 7 groups of 6 animals and tested at one of 7 times of day 4 hours apart . the same groups of animals were retested a week later with the same protocol , except that the animals initially tested first during the light phase of the light - dark cycle were tested first during the dark phase , and vice versa . reaction time ( average of the three methods ) was longest at the transition from light to darkness and shortest a few hours before the transition from darkness to light . this suggests greater pain sensitivity late in the dark phase , which is in agreement with previous studies in rats and golden hamsters . another study in rats suggested the occurrence of greater sensitivity during the light phase , but this was probably an artifact of the experimental procedure , as only two time points during the day were reported . a study on mice suggested the occurrence of two daily peaks in sensitivity , one during the light phase and one during the dark phase . the amplitude of the daily variation in latencies was much smaller in that study than in ours , and it is possible that random oscillations were interpreted as a daily rhythm . daily rhythmicity of nociception in rats the figure shows the daily variation in reaction time to nociceptive stimulation . the horizontal bar at the top indicates the timing of the light - dark cycle . sensitivity to pain is highest late in the dark phase of the light - dark cycle and lowest at the light - dark transition . we , the authors declare that we have not received funds from any agency or organization for carrying out this work . carried out the study cv  carried out the replicate study nm and sjp  evaluated the data statistically