Nervous system, endocrine, and/or immune system difficulties may affect one of the less measurable causes of bipolar disorders: disturbances in body rhythms. As discussed earlier, the hypothalamus is the link between the nervous and endocrine systems. The nervous system is also associated with the immune system. Because of this, it is possible that the hypothalamus influences the immune system. The combination of these systems may alter the body's biochemistry. Those changes could then contribute to shifts in body rhythms such as the circadian, seasonal, and social rhythms.
Circadian Rhythm and Bipolar Disorder
The term circadian rhythm refers to the approximately 24-hour cycle of the body. This cycle is determined by the amount of light that the hypothalamus in the brain senses in a day-night cycle. Both brain wave activity and hormone production are coupled to this cycle. When the circadian rhythm is upset (as can be the case with jet lag and sleep problems) mood disturbances can result. It is known that in some people sleep deprivation causes mania, whereas in others it can alleviate the symptoms of depression. Thus, regulation of circadian rhythm is important for managing bipolar symptoms and mood episodes. Dysregulation of this system is typically experienced as a powerful source of stress.
Major Depressive Disorder with a Seasonal Pattern and Bipolar Disorder
Seasonal rhythms are similar to circadian rhythms but are longer in duration. These are determined by the amount of daylight experienced within a given season. Dysregulation of seasonal rhythms has been linked with Major Depressive Disorder with a Seasonal Pattern (previously known as Seasonal Affective Disorder or "winter depression"). These individuals begin to feel increasingly depressed as the amount of light disappears during the winter. Then these individuals experience a lift in mood as springtime (and more light) approaches. During other times of the year, people with this seasonal pattern experience "normal" mental health.
Social Rhythms and Bipolar Disorder
Both circadian and seasonal rhythms can affect individuals' social rhythms. The social rhythm is made up of a daily routine such as waking up at a specific time, going to school or work, and interacting with family members, friends, peers and colleagues. Even healthy people can experience mood changes when their social rhythms are disturbed by insomnia, seasonal changes or work schedules. It is no great leap to see that if someone is vulnerable to bipolar disorder, a change in their body rhythms might be sufficient stress to bring about bipolar symptoms.
When considering the possible causes of bipolar disorders, it is necessary to keep in mind the complex nature of mood issues. It is likely that that the problem begins in multiple biologic systems. These may include the nervous, endocrine, and immune, as well as the genetic machinery that regulates these systems. For example, a gene important in the function of the hypothalamus may become mutated or infected by a retrovirus. This change then leads to deregulation of neurotransmitters, hormones, and/or immune components. The resulting change is measurable as biochemical imbalances in either the brain or body. However, these biochemical imbalances are not necessarily causing the underlying dysfunction. They may simply be themselves symptoms or links in a chain of causes that lead to bipolar disorder.