Some survivors tire quicker and age faster.

• Position: Breast Cancer Patients

The persistent fatigue that plagues one out of every three breast cancer survivors may be caused by one part of the autonomic nervous system running in overdrive, while the other part fails to slow it down. That imbalance of a natural system in the body appears linked to the tiredness and exhaustion that can burden cancer patients as much as a decade after their successful treatment.

The effect is so great, researchers indicate, that it may be a sign of accelerated aging in fatigued patients, causing them to seem as much as 20 years older compared with patients who are not fatigued, note researchers from Ohio State University, Columbus.

Christopher Fagundes, a postdoctoral fellow at the Institute of Behavioral Medicine Research, and Janice Kiecolt-Glaser, professor of psychiatry and psychology, were looking for a biomarker, a signal that could point to the initial cause of this fatigue. Their target was the autonomic nervous system, that part of the body that controls unconscious activities like breathing, heartbeat, digestion, and such, which earlier research had indicated might play a role.

The autonomic nervous system has two main parts--the sympathetic and the parasympathetic. The former is responsible for what has become known as the fight-or-flight...