Clinical depression and bipolar disorder are classified as separate illnesses, but psychiatry is increasingly viewing them as part of an overlapping spectrum that also includes anxiety and psychosis.
In a 2003 study, Akiskal and Judd re-examined data from the landmark Epidemiological Catchment Area study from two decades before. The original study found that .08 percent of the population surveyed had experienced a lifetime manic episode (the diagnostic threshold for bipolar I) and .05 a hypomanic episode (the diagnostic threshold for bipolar II). But by tabulating survey responses to include criteria below the diagnostic radar, such as one or two symptoms over a short time period, the authors of the study recalculated the data to arrive at an additional 5.1 percent of the population, adding up to a total of 6.4 percent of the entire population who could conceivably be thought of as having bipolar disorder.
There is also a case that clinical (unipolar) depression can be bipolar disorder waiting to happen. In a 2005 study, Jules Angst and his colleagues at Zurich University tracked 406 patients with major mood disorders over a 20-year period. Of 309 patients presenting with depression, 121 (39.2 percent) eventually manifested as bipolar (24.3 percent to bipolar I, 14.9 percent to bipolar II). In all, more than 50 percent of the study population turned out to have bipolar disorder. (See The Mood Spectrum.)
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.
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". 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.
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 postpartum depression (PD) puerperal psychosis (PP). Ian Jones of the Department of Psychological Medicine in Cardiff is researching this area.
Bipolar disorder is often complicated by co-occurring alcohol or substance abuse. Traditionally this has been viewed as an attempt by patients to self-medicate the condition. More recently, some have doubted if this is an entirely accurate description. Cannabis in particular can alleviate symptoms of depression and may also have a mood stabilizing component in bipolar disorders, but the random titration of drug abusers usually does do more harm than good. There is growing evidence, however, that carefully titrated dosage of delta-9-THC tincture, taken sublingually, may prove of some benefit when taken with other mood stabilizer medications. In some cases, the substance abuse seems to begin before the onset of bipolar disorder, which is difficult to reconcile with the idea of self-medication (at least initially). Nicotine addiction is very common in people with bipolar disorder, and in the view of some, may be an active precursor to mature onset of both bipolar affective disorder and other forms of clinical depression in general.
Drugs like adderall, Ritalin or any stimulant can produce mania, but often times this is not actually bipolar disorder, but a singular manic episode. This is valid according to the DSM.