What physical body function do stimulants most commonly increase?

Stimulants are substances that temporarily increase alertness and energy levels in individuals. One of the primary physical effects of stimulants is an increase in heart rate. This occurs because stimulants act on the central nervous system, often leading to increased levels of certain neurotransmitters, such as norepinephrine and dopamine. As a result, the heart responds by beating faster, which can provide a sense of heightened energy and focus.

In contrast, the other options reflect body functions that are typically not associated with the effects of stimulants. Muscle relaxation, digestion, and sleepiness tend to be influenced by other types of substances, such as depressants or sedatives, rather than stimulants which are meant to energize and invigorate bodily functions. Thus, the specific increase in heart rate is a defining characteristic of how stimulants operate within the body's systems.