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
 backgroundmany behavioral and physiological variables exhibit daily rhythmicity . 
 few investigations of the daily rhythmicity in nociception have been conducted , and conflicting results have been obtained . 
 the present study evaluated the daily rhythmicity in nociception in wistar rats.methodsnociception was investigated 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 . 
 separate groups of rats were tested in 4-hour intervals for 24 hours.resultsthere was clear daily variation in response latency . 
 reaction time was shortest a few hours before lights - on and longest at the light - dark transition.conclusionnociception exhibits robust daily rhythmicity in rats . 
 sensitivity to pain is highest late in the dark phase of the light - dark cycle and lowest at the light - dark transition . 
Background
Methods
Results and discussion
Conclusion
Competing interests
Authors' contributions
Acknowledgement