Morning light exposure for the treatment of winter depression: the one true light therapy?

The time of day when bright light should be administered to treat winter depression has become controversial. Results from clinical trials suggest that light given at any of several times of day can produce an antidepressant response. However, the basis for this effectiveness has been questioned and theoretical issues remain unresolved. It appears that individuals differ in their sensitivity to time of day as well as to dose. Unfortunately, important experimental details such as pretreatment severity of depression and order effects have not been taken into account, and this has made definitive conclusions impossible to reach. In order to understand the mechanism(s) by which light treatment works, we must discuss and employ common criteria for study design and data analysis. For clinical purposes, it currently seems that time of treatment can be adjusted to accommodate the physiological responsiveness and lifestyle requirements of individual patients.