The depression experienced by people having a bipolar disorder is similar to a clinical depression. The symptoms include persistent feelings of anxiety, sadness, anger, guilt, hopelessness, and isolation, appetite and sleep disturbances, fatigue, escapism, lack of interest on things once enjoyed, problems concentrating,  indifference or apathy, self-loathing, social anxiety or shyness, chronic pain, irritability, and suicidal tendencies.

When talking about disability, loss of productive years, potential suicidal tendency and duration, the periods of depression in a bipolar disorder are widely recognized today as the most severe problem of an individual. Sometimes, the mania periods are more disruptive and noticeable to other people.

Some types of serious depression are also accompanied by psychosis symptoms. It includes hallucinations (seeing, hearing, or sensing stimuli perceive to be there although it is not), delusions (false beliefs of a person that aren't subject to contradictory evidence or reason and aren't explained by a cultural concept), and escapism (creating diversions mentally to escape from unpleasant and perceived phases of stress).

These people may be also affected with paranoid thoughts that a powerful entity is monitoring or persecuting them. Some may think that those people who are close to them are conspiring and bullying against them. Greater levels of anxiety are felt without a cause. They felt that their family or friends are giving them up or leaving them. Unusual and intense religious beliefs is also present, thus some people strongly insists that historic and great missions are given to them to be accomplished believing that they have supernatural powers. Delusions can be more distressing in depressions. It is manifested in the form of a strong guilt for things that are supposedly done wrong inflicting the lives of other people. Numerous conflicting theories are created considering different causes associated with bipolar disorder. Nevertheless, nothing is accepted widely as correct.

The primary nature of a bipolar disorder is flux. It is the state of mental operation wherein a person is immersed fully on what she or he is doing, accompanied with a feeling of full involvement, energized focus, and success during the activity process. The biological markers of a bipolar disorder include mood, energy, sleep, activity, and thought. The bipolar disorder's diagnostic subtypes are sometimes static snapshots of the descriptions of the continual changes of an illness.

According to the US NIMH (United States National Institute of Mental Health), there is no predetermined cause of a bipolar disorder. However, lots of factors are acting together to create the illness. This disorder also runs in families, thus most researchers are searching for a specific gene which increases the possibility of developing the disease. Most findings suggest that the disorder is caused by multiple genes similar to other mental diseases.