CME: Light Therapy: Why, What, for Whom, How, and When (And a Postscript about Darkness).

Light therapy has become established as an evidence-based treatment for Seasonal Affective Disorder. Light impacts the timing and stability of circadian rhythms as expressed in sleep, mood, alertness, and cognition. Forty years of clinical trials and open treatment have led to guidelines for patient selection, using light alone or in combination with antidepressants (or lithium for bipolar depression).