Abstract
STUDY OBJECTIVES: Insufficient sleep and chronic pain are public health epidemics. Sleep loss worsens pain and predicts the development of chronic pain. Whether previous, acute sleep loss and recovery sleep determine pain levels and duration remains poorly understood. This study tested whether acute sleep deprivation and recovery sleep prior to formalin injection alter post-injection pain levels and duration. METHODS: Male Sprague-Dawley rats (n = 48) underwent sleep deprivation or ad libitum sleep for 9 hours. Thereafter, rats received a subcutaneous injection of formalin or saline into a hind paw. In the recovery sleep group, rats were allowed 24 h between sleep deprivation and the injection of formalin. Mechanical and thermal nociception were assessed using the von Frey test and Hargreaves' method. Nociceptive measures were performed at 1, 3, 7, 10, 14, 17 and 21 days post-injection. RESULTS: Formalin caused bilateral mechanical hypersensitivity (allodynia) that persisted for up to 21 days post-injection. Sleep deprivation significantly enhanced bilateral allodynia. There was a synergistic interaction when sleep deprivation preceded a formalin injection. Rats allowed a recovery sleep period prior to formalin injection developed allodynia only in the injected limb, with higher mechanical thresholds (less allodynia) and a shorter recovery period. There were no persistent changes in thermal nociception. CONCLUSION: The data suggest that acute sleep loss preceding an inflammatory insult enhances pain and can contribute to chronic pain. The results encourage studies in a model of surgical pain to test whether enhancing sleep reduces pain levels and duration.