According to previous studies, researchers have theorized that increased activity of the HPA axis is associated with depressed patients with typical melancholic features. Melancholia refers to the feelings of anhedonia (absence of pleasure from activites that would normally be thought of as pleasurable), insomnia (inability to sleep), guilt, and psychomotor changes. On the other hand a decrease in activity of the HPA axis may be associated with the atypical features of depression.