Is seasonal affective disorder a variant of atypical depression? Differential response to light therapy

Similar symptomatology has been described for both seasonal affective disorder (SAD) and atypical depression. For example, hyperphagia, hypersomnia, and intense lethargy are common to both, suggesting that they might be subtypes of the same disorder. If SAD and atypical depression are different manifestations of the same underlying pathophysiology, treatment effective for one might also benefit the other. Bright artificial lights (2500 lux, 6-8 a.m. and p.m.) were significantly less effective in treating eight patients diagnosed as having atypical depression without a seasonal pattern than 25 SAD patients. Differential treatment outcome suggests that SAD and atypical depression are separate disorders.