Male prairie voles display cardiovascular dipping associated with an ultradian activity cycle.

Abstract Mammals typically display alternating active and resting phases and, in most species, these rhythms follow a circadian pattern. The active and resting phases often are accompanied by corresponding physiological changes. In humans, blood pressure decreases during the resting phase of the activity cycle, and the magnitude of that “nocturnal dipping” has been used to stratify patients according to the risk for cardiovascular disease. However, in contrast to most mammals, prairie voles ( Microtus ochrogaster ) have periods of activity and rest that follow an ultradian rhythm with period lengths significantly