Recent data has highlighted the association of a bipolar disorder (BD) with an increased risk of post-partum psychosis and depression. It is suggested that genetic- and environmental-induced decrease in the levels of melatonin in BD contributes to post-partum disorders. Melatonin may also have some efficacy in the treatment of BD, especially in decreasing the side-effects associated with lithium and the neuroleptics. It is proposed that the optimization of melatonin levels, perhaps in conjunction with optimized vitamin D3 level, would decrease post-partum psychosis and depression associated with BD.