Papers of the Week

Sleep and muscle injury-related pain are in negative relationship, and sleep extension may be a favorable countermeasure. In response to muscle injury, an adaptive sleep response has been described in rats, characterized by an increase in total sleep time (TST) and non-rapid eye movement (NREM) sleep. This study examined the effects of photoperiod lengthening (a model of sleep prolongation in rats) on the sleep characteristics of muscle-injured rats, and whether this lengthening could benefit injury-induced mechanical hyperalgesia using the Von Frey test. Switching from the conventional 12:12 light-dark (LD) photoperiod (light on: 08:00-20:00) to LD 16:8 (light extended to 24:00) gives rats an extra window of sleep. Our results show higher TST and NREM sleep times in LD 16:8 LD 12:12 injured rats during the 4 hours of light lengthening for 7 days post-injury, showing the efficiency of photoperiod lengthening to increase sleep time in injured rats. In addition, a cumulative effect with the adaptive sleep response to muscle injury occurred with higher TST and NREM sleep times in LD 16:8 injured non-injured rats during the dark period, reflecting the high need for sleep after the injury. Greater stability and higher relative delta power of NREM sleep during the extended light period were also observed in injured rats. Finally, the extended photoperiod limits the muscle injury-induced mechanical hyperalgesia for 13 days and allows faster recovery of the baseline mechanical threshold. This is associated with reduced pro-inflammatory cytokines levels in the hippocampus, a brain structure involved in painful processing. Our results demonstrate the efficiency of photoperiod lengthening to induce sleep extension in rats after muscle injury. Furthermore, there is a cumulative effect of photoperiod lengthening adaptive sleep response to muscle injury on sleep time, suggesting a significant need for sleep in injured rats. In addition, the photoperiod lengthening limits muscle injury-induced mechanical hyperalgesia and allows faster recovery of baseline paw withdrawal threshold, associated with a decrease in pro-inflammatory cytokines in the hippocampus. These results suggest that photoperiod lengthening could potentially be used as a non-pharmacological treatment for mechanical hyperalgesia after muscle injury.