Nervous system, endocrine, and/or immune system difficulties may conspire to 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. Given that the nervous system is also associated with the immune system, it is possible that the hypothalamus exerts its effect on the immune system as well. Thus, the combination of these systems can alter body biochemistry, contributing to shifts in body rhythms such as the circadian, seasonal, and social rhythms.
Circadian Rhythm and Bipolar Disorder
The circadian rhythm is the 24-hour cycle of the body, the exact length of which is determined by the amount of light that the hypothalamus senses in a day-night cycle. The name "circadian" refers to a period of time that is "around a day long". Clear patterns of 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.
Seasonal Affective Disorder (SAD) and Bipolar Disorder
Similar to circadian rhythms but longer in duration are seasonal rhythms. These are determined by the amount of daylight experienced within a given season. Dysregulation of seasonal rhythms has been linked with Seasonal Affective Disorder (SAD; also known as winter depression). SAD-affected individuals begin to feel increasingly depressed as the amount of light disappears during the winter. Their depression lifts as springtime approaches and the days lengthen. During times of the year with long days, these people typically experience no undue mood disturbance.
Social Rhythms and Bipolar Disorder
Both circadian and seasonal rhythms can affect individuals' social rhythms. The social rhythm comprises 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 susceptible to bipolar disorder, a change in their body rhythms might constitute sufficient stress to precipitate bipolar symptoms.
When considering the possible causes of bipolar disorders, it is necessary to bear in mind the complex nature of mood polarity. It is likely that that the problem originates in multiple biologic systems - the nervous, endocrine, and immune systems with a basis in 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 leading to deregulation of neurotransmitters, hormones, and/or immune components. The resulting change is measurable as biochemical imbalances in either the brain or body but these biochemical imbalances are not necessarily causing the underlying dysfunction, but instead may simply be themselves symptoms or links in a chain of causes that lead to bipolar illness.