Failure of extraocular light to facilitate circadian rhythm reentrainment in humans

Although extraocular light can entrain the circadian rhythms of invertebrat es and nonmammalian vertebrates, almost all studies show that the mammalian circadian system can only be affected by light to the eyes. The exception is a recent study by Campell and Murphy that reported phase shifts in human s to bright light applied with fiber-optic pads behind the knees (popliteal region). We tested whether this extraocular light stimulus could accelerat e the entrainment of circadian rhythms to a shift of the sleep schedule, as occurs in shift work or jet lag. In experiment 1, the sleep/dark episodes were delayed 8h from baseline for 2 days, and 3h light exposures were timed to occur before the temperature minimum to help delay circadian rhythms. T here were three groups: (1) bright (about 13,000 lux) extraocular light fro m fiber-optic pads, (2) control (dim light, 10-20 lux), and (3) medium-inte nsity (about 1000 lux) ocular light From light boxes. In experiment 2, the sleep/dark episodes were inverted, and extraocular light was applied either before the temperature minimum to help delay circadian rhythms or after th e temperature minimum to help advance rhythms. Circadian phase markers were the salivary dim light melatonin onset (DLMO) and the rectal temperature m inimum. There was no evidence that the popliteal extraocular light had a ph ase-shifting effect in either experiment. Possible reasons for phase shifts in the Campbell and Murphy study and not the current study include the man y differences between the protocols. In the current study, there was substa ntial sleep deprivation before the extraocular light was applied. There was a large shift in the sleep/dark schedule, rather than allowing subjects to sleep each day from midnight to noon, as in the Campbell and Murphy study. Also, when extraocular light was applied in the current protocol, subjects did not experience a change from sleeping to awake, a change in posture (f rom lying in bed to sitting in a chair), or a change in ocular light (from dark to dim light). Further research is necessary to determine the conditio ns under which extraocular light might produce phase shifts in human circad ian rhythms.