Due to the potential for M100907 to improve sleep architecture in general and slow wave sleep in particular, sleep will be studied using polysomnographic methods before and after randomization. Sleep disturbance is also a well-known side effect of SSRI's, and the addition of M100907 may improve sleep disturbance related to the affective disorder itself or to iatrogenic SSRI effects on sleep. Examination of improvement in sexual side effects, gastrointestinal side effects, headache frequency, and anxiety will also be investigated in the comparison of addition of M100907 versus placebo in escitalopram treated subjects. DNA samples will be collected for neurotransmitter-related genetic polymorphisms for covariance.