Relative coordination to unknown "weak zeitgebers" in free-running blind individuals

Light is the primary synchronizer of the human biological clock. In more than half of those blind individuals who completely lack light perception, the absence of photic input to the hypothalamic circadian pacemaker results in rhythms that free-run (blind free-runners [BFRs]) with a period typically greater than 24 h. The remainder are entrained, although sometimes at an abnormal phase angle. It is presumed that weak as-yet-to-be-identified time cues provide the necessary resetting stimulus in these entrained individuals. These weak zeitgebers might be expected to modulate the observed circadian period in blind people who are not actually entrained by them. The authors report here the results from 5 BFRs (average linear regression period +/- SD of 24.31 +/- 0.06 h) who had high-resolution (many and frequent) phase assessments. All 5 subjects demonstrated a similar and reproducible pattern of changes in observed period (period response curves) indicative of relative coordination. The precise shape of the period response curve to weak zeitgebers has implications for the entrainment of BFRs using exogenous melatonin administration or other nonphotic stimuli. Sighted individuals may also be affected by such weak zeitgebers, which may be obscured by the stronger light/dark cycle.