The purpose of this study is to examine the effects of (a) quetiapine XR and (b) mirtazapine on sleep when given as an antidepressant (monotherapy). We hypothesize that (a) quetiapine XR has an immediate and lasting positive effect on sleep in depressed patients which does not differ from the impact of mirtazapine on sleep in this group of patients; (b) in the context of a secondary objective, we expect an antidepressant effect of quetiapine XR which is equivalent to that of mirtazapine.