In mid-2003, a twin study was published concerning environmental factors and bipolar disorder. The bipolar twin was found to be far more affected by changes in sunlight. Longer nights resulted in mood and sleep-length changes far greater than the healthy twin. Sunny days also did more to improve mood. In fact, natural light in general was found to have a profound positive effect upon the well-being of the bipolar twin (Hakkarainen, 2003).

Paradoxically, in the 2004 publication of a study using Tel Aviv's public psychiatric hospitals, it was found that "Admission rates of bipolar depressed patients increase during spring/summer and correlate with maximal environmental temperature" (Shapira, 2004). Unipolar depressed patient admission had no such correlation. High temperature points in the month, as well as high temperature months, were found to be correlated with depressive episodes in admissions.

Bipolar disorder and childbirth:

For many women with depression or bipolar disorder the postpartum period is a period of risk for developing illness. Episodes of bipolar disorder that follow childbirth are traditionally called puerperal psychosis (PP). Dr. Ian Jones of the Department of Psychological Medicine in Cardiff is researching this area.