THE EFFECT OF CAFFEINE ON THE HEART RATE VARIABILITY. Caffeine is a plant element found in beverages, drugs, and food. It is a stimulant used to make people more alert. The consumption of drinks with caffeine content acts as a stimulant to the automatic nervous system (ANS) (Hassett & Radvanski, 2013, p.3). The blood pressure, heart rate (HR), among other similar cardiovascular activities, have been widely investigated. ?evertheless, there are rare studies concerning the impact of caffeine on heart rate variability (HRV).