There has been considerable skepticism about the value of light therapy for people with seasonal depression, despite a very compelling research literature. Perhaps this might, in part, be because the way that light might affect mood has not been clear. In a recently published animal study, change in light exposure increases stress hormones, depresses mood, and impairs learning through changes light sensitive cells in the retinal ganglion (the inner most layer of the eye).
The authors of this paper changed the usual mouse cycle of 12 hours of light and 12 hours of dark to 3.5 hours of light and 3.5 hours of dark. Mice with the shortened cycle had similar sleep amount and quality as those with the normal cycle.
Compared with mice with a 24 hour cycle, the mice with a 7-hour cycle had higher stress hormone levels and had poor performance on tests of mouse cognition. They also had a pattern of “learned helplessness” in another test. Learned helplessness is a pattern of apathetic behavior that is felt to be a model for human depression.
Seven hour cycle mice had greater brain activity in brain areas associated with memory and emotion (the hippocampus and the amygdala and habenula in the limbic area).
It was shown that this was the result of the activity of certain cells in the eye.
Chronic fluoxetine (Prozac) treatment reversed the learning deficits and normalized stress hormone level
Patients and families may be relieved to learn that there is a direct physiological pathway because of some public skepticism about the value of light therapy.
LeGates TA et al. Aberrant light directly impairs mood and learning through melanopsin-expressing neurons. Nature 2012 Nov 22; 491:594.