Diurnal variations have been established as a symptom of severe depression where a person experiences all symptoms of severe depression during the day and the symptoms magically disappear in the evening only to return the next day. This is the action of diurnal variations. There are a number of studies that attempt to offer an explanation of how this phenomenon works. Many studies believe that diurnal variations involve interactions between circadian rhythms and mood regulation. However, the mechanism of action of diurnal variations is still not well understood. A theory is presented of how and why diurnal variations functions; that it is activated by a specific part of the brains immune system.