The reason behind your lethargy, sleep disruptions and fatigue could be bipolar personality disorder - a mental illness causing changes in mood and thinking. People with this mental disorder experience acute depression which can even be life threatening. Understanding the causes of bipolar disorder therefore, can be a great help in learning to deal with the illness.  

First, let us take a close look at the nature of bipolar personality disorder. This illness is an emotional disorder causing frequent mood shifts. Patients with this disorder may experience any emotional state, sadness or happiness to the extreme level.  Unfortunately, there are no warning signs and the disorder just comes out of the blue.  

Also, since medical science has not yet developed a definitive method to detect the illness, understanding the causes of bipolar disorder becomes difficult. Doctors usually rely on the symptoms for diagnosis, which is done after a week of careful observation. The symptoms of the problem are cyclic. Patients may have complex mental issues frequently or rarely. Psychiatrists classified the symptoms into two: depressive and manic.  

Depressive symptoms include lack of self esteem, social withdrawal, incapability to control emotion, indigestion, inability to have happiness. In the case of manic disorder, symptoms include delusions, weakness to drugs and alcohol, engaging in dangerous activities and rapid speech. If you detect any of these symptoms in someone‚Äôs behavior or attitude then, that person is likely to be afflicted with bipolar disorder.  

Trying to control or subdue these disruptive symptoms is not possible without understanding the causes of bipolar disorder.  This is a daunting task because even after many years of painstaking research through surveys of patients with light or acute symptoms, doctors have gotten only inconclusive results. So nobody has been successful in determining the exact causes of bipolar disorder.  

Scientific evidence indicates that heredity or genetic factors can trigger the disorder.  Studies have shown that children inherit this disorder from parents with the same problem. Also, identical twins, because of the same genes, can have bipolar disorder. There is a slight chance of developing the symptoms of the disorder among siblings.  

Scientific evidence also points to some environmental factors such as steroid consumption, lack of sleep and even the weather as triggers for the appearance of bipolar symptoms.  Add to this the fact that psychotherapists are not ruling out psychological and hormonal factors as causes of bipolar disorder, and then you can see how confusing the picture has become. Data from family history and friends provide another clue as to the possible causes of this disorder.

Whatever the causes of bipolar disorder are, once the problem is detected, appropriate treatment and medication must be started immediately to obtain the best results as well as the possibility of a complete cure. Neglecting the problem always result in the patient getting worse and unmanageable,  

Reference

  http://www.bipolardepressioninfo.com/ms/guides/causes_of_bipolar_disorder/main.html

http://www.emedicinehealth.com/bipolar_disorder/article_em.htm