We study the heartbeat activity of healthy individuals at rest and during exercise. We focus on correlation properties of the intervals formed by successive peaks in the pulse wave and find significant scaling differences between rest and exercise. For exercise the interval series is anticorrelated at short-time scales and correlated at intermediate-time scales, while for rest we observe the opposite crossover pattern—from strong correlations in the short-time regime to weaker correlations at larger scales. We also suggest a physiologically motivated stochastic scenario to provide an intuitive explanation of the scaling differences between rest and exercise.