Mental fatigue caused by prolonged cognitive load associated with sympathetic hyperactivity.

Background: It is known that chronic fatigue is associated with sympathetic hyperactivity. However, the relationship between autonomic function and mental fatigue caused by a prolonged mental load in healthy humans is still unclear. Thus, in order to clarify the mechanisms underlying mental fatigue, we examined the association between mental fatigue and autonomic functions.

Methods: The study group comprised 10 healthy participants. To induce mental fatigue, participants performed mental tasks, which consisted of the advanced trail making test, kana pick-out test and mirror drawing test, for 8 hr, corresponding to a normal work day. Autonomic functions were measured by accelerated plethysmography before and after the fatigue-inducing mental tasks. As a control, the same participants completed an 8-hr relaxation session 4 weeks before the fatigue session.

Results: After the 8-hr relaxation session, low-frequency component power (LF), high-frequency component power (HF) and low-frequency component power/high-frequency component power ratio (LF/HF ratio) were not changed from baseline. In contrast, after the fatigue session, the HF and LF/HF ratio were significantly changed from baseline; specifically, the HF was lower and LF/HF ratio was higher as compared to those after the relaxation session.

Conclusions: Sympathetic hyperactivity based on decreased parasympathetic activity is associated with mental fatigue induced by prolonged cognitive load.