There are several possible physiological causes of Circadian Rhythm Sleep-Wake Disorders (CRSWDs), including lack of sensitivity to light, over-sensitivity to light, deficiencies in the ipRGC cells of the retina, lack of melatonin production, long elimination time of melatonin, long intrinsic circadian period, differences in timing of sleep relative to internal circadian rhythms, differences in tolerance to phase mismatch, etc. The specific cause likely depends on the individual patient. One would expect that some treatments work for certain causes, other treatments for others. Today a diagnosis is made solely on the symptoms, the natural sleep hours, and not the underlying cause. As a result, treatment is often ineffective.