When does the heart rate typically increase?

The heart rate typically increases when sympathetic fibers are stimulated because this part of the autonomic nervous system is responsible for the body's "fight or flight" response. When these fibers are activated, they release catecholamines such as adrenaline, which bind to receptors in the heart and cause the heart muscles to contract more forcefully and more frequently. This leads to an increase in heart rate, enabling the body to respond to stress or increased demand for blood flow, such as during physical activity or moments of excitement and stress.

In contrast, during rest or sleep, the parasympathetic nervous system is more active, which tends to decrease heart rate. Similarly, lying down or being in a relaxed state generally does not require the heart to pump blood as vigorously, thus resulting in a lower heart rate. The sympathetic stimulation is what directly causes the heart to speed up, making it the correct answer in this context.