Many diseases show patterns of temporal variation that remain unexplained. We hypothesize that oscillation of autonomic balance over different time intervals plays a role in these variations. In the Darwinian struggle, organisms must perform various functions related to fitness such as survival, energy acquisition, and reproduction in a prioritized fashion. Autonomic systems enable differential allocation of effort to various functions under different conditions, some of which vary in patterns that are predictable such as circadian, menstrual, seasonal, and life cycles. For many species, daytime functions such as seeking food and avoiding predators are achieved with sympathetic activity while night-time functions such as internal processing are achieved with parasympathetic activity. For organisms that maintain thermal homeostasis year-round, cooler temperature seasons may demand sympathetic bias to drive adaptive thermogenesis. Reproduction may necessitate autonomic shift to sympathetic bias during the luteal phase and pregnancy to modulate immune balance towards a more tolerant Th2 bias. Many diseases including infectious, cardiovascular, inflammatory, pulmonary, metabolic, fertility, oncologic, and neurologic conditions also show variation in prevalence over these cycles. The co-variation of autonomic balance and diseases over time intervals suggests that autonomic balance, by its direct effects as well as its indirect effects through modulation of T helper immune balance, plays an under-recognized role in diseases. The theory is extended to the co-variation of autonomic balance and diseases over the lifespan. Termination of organisms during senescence, achieved by emergence of autonomic imbalance and other systemic dysfunctions, is explored from a Darwinian perspective.
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