Circadian rhythms are physical, mental, and behavioral changes that follow 24 hr cycle. The production of antioxidants and protective enzymes, have been reported to be regulated or expressed in rhythmic fashions. Thus, oxidative stress seems to have a circadian rhythm connection. Oxidative stress has been implicated in aging and neurodegenerative diseases. Reactive oxygen species have role in pain associated with peripheral nerve injury. Increase in oxidative stress as in aging cause profound decline in opioid system at supraspinal level than at spinal level and also the nociceptive threshold. Oxidative stress decreases the nociceptive threshold and also opioid receptor function leading to decreased opioid induced antinociceptive effects. Thus it is hypothesised that the circadian pattern of nociception is due to oxidative stress level in the brain areas.

Keywords: Circadian Rhythm; Nociception; Oxidative Stress