Precision Light for the Treatment of Psychiatric Disorders

Box 1

Generally, to demonstrate that an environmental variable acts as an entrainment cue for the circadian system, four criteria must be met:

(1) The absence of all other time cues. In the absence of time cues, self-sustaining, but unentrained, circadian rhythms “free-run” with a period deviating slightly from 24 hours; the circadian rhythm under investigation must be free-running with an independent period prior to the administration of the entraining signal and must resume its free-running period after the signal is removed.

(2) The control of period. Following exposure to the entrainment signal’s environmental cycle, the endogenous period of the investigated circadian rhythm must adjust itself so that it is aligned to the period imposed by the entrainment signal.

(3) A stable phase relationship. A stable and reproducible phase relationship must emerge between the timing of the circadian rhythm investigated and the timing of the entrainment signal, whereby the resulting phase of the circadian rhythm is independent of clock-time and solely dependent on the time cue imposed by the entrainment signal.

(4) The control of phase. When the entrainment signal is removed, the resulting circadian rhythm must free-run with a phase determined by the entrainment signal and not by the free-running rhythm prevailing prior to entrainment.