Light Therapy and Psychiatry(42380A)

Circadian (24-hr) rhythms are driven by an endogenous pacemaker. In constant conditions free of external time cues (Zeitgebers), the pacemaker and its driven rhythms “free run” at an intrinsic period close to, but not precisely, 24 hr (1). Twenty-four hour precision is accomplished mainly by entrainment, or synchronization, to the 24-hr light-dark cycle through daily phase shifts (a shift to either an earlier or a later time of the day) that result from exposure to light (2-4). These phase shifts vary in magnitude and direction according to when the light exposure occurs, a relationship that can be described by a phase response curve (PRC). The basic features of a PRC are that phase delays (shifts to a later time) occur in response to light exposure during the first part of the subjective night and that phase advances (shifts to an earlier time) occur in response to light exposure during the second part of the subjective night. (In constant dark conditions, subjective night refers to the sleep phase of diurnal animals and the activity phase of nocturnal animals.) During the subjective day, light exposure has relatively little effect. Generally, the middle of the night is when phase shifts are of the greatest magnitude and when there is an inflection point in the PRC that separates delay responses from advance responses. In nature, only moonlight illuminates the high-amplitude portions of the PRC that occur in the middle of the night. Entrainment to the 24-hr day is thus provided by exposure to sunlight at dawn and at dusk, resulting in a phase advance and a phase delay, respectively, each day. These phase shifts compensate for each other and their net effect must also compensate for the pacemaker's intrinsic (free-running) period.