Light therapy for winter depression (seasonal affective disorder) was first developed and explored about 35 years ago at the US National Institute of Mental Health (NIMH). It was a momentous leap from basic biological thinking, which looked for causes of behavior within living things. Instead, the new focus of attention was on the environment－an emphasis inspired by the longstanding observation that certain animals (“photoperiodic” creatures) reproduced or hibernated on a seasonal cycle related to the amount of outdoor light available.
Further, the NIMH group had recently discovered that bright light immediately suppressed human production of melatonin by the pineal gland at night, and melatonin was well known to be a seasonal signaler in photoperiodic animals. This was the first antidepressant ever to be discovered based on an established biological framework! Drug treatments for depression, by contrast, were discovered by chance, and had been developed originally for other purposes.
Light therapy for winter depression set off an explosive international research effort. The psychiatric establishment was extremely skeptical. Did seasonal affective disorder (SAD) really exist, or was it fiction that appealed to the uninformed public? Was light therapy merely a placebo? How could one test light against a “blind” placebo? Would the effect last? Was it specific to winter depression, or could it be applied across the board for depressive disorders?
It was unclear at the start how, or whether, the adjustment of circadian rhythms affected the treatment response. However, the response was often quick and dramatic, with remissions obtained within a few days of daily bright light exposure.
After years of clinical trials, circadian rhythm involvement did become clear. In winter, with later sunrise, rhythms tended to drift later. Intervening with early morning artificial light shifted the rhythms earlier, while patients experienced the lift from depression. Light at other times of day seemed to have partial benefit, but was not as powerful as morning light; perhaps it was the activating effect of light exposure that reduced some of the depressive symptoms.
The momentous clinical discovery that light affected depression laid the foundation for multiple applications of light therapy far afield from SAD. And it was the inspiration for the formation of the Center for Environmental Therapeutics!