A reader asks:

 When you say “optimal timing” of light therapy, what do you mean? What makes the timing “optimal”?


The timing of daily light exposure determines — along with light intensity and session duration — the strength of the antidepressant effect. We have learned that when light exposure causes the internal circadian clock to shift earlier, the antidepressant effect is enhanced. The sensitivity of the clock to light is a strict function of the time of day of exposure. The Morningness-Eveningness Questionnaire (AutoMEQ) at www.cet.org was designed to estimate optimal timing for each individual. Such timing can differ by several hours from one person to the next.