During which condition would a patient most likely exhibit sinus tachycardia?

Sinus tachycardia is typically characterized by an increased heart rate that arises from the sinus node in response to various physiological stimuli, most commonly demand for higher cardiac output. After physical activity is a prime scenario for the onset of sinus tachycardia, as the body requires more oxygen and nutrients for the working muscles. The heart naturally responds by increasing its rate to meet this heightened demand.

In contrast, deep sleep typically leads to a state of decreased heart rate due to the parasympathetic nervous system being more active during this phase, which promotes slower heart rhythms. Resting without any stimuli similarly decreases heart rate as there is no need for increased cardiac output. Atrial fibrillation, on the other hand, is an irregular heart rhythm that can coexist with tachycardia but does not originate from the sinus node, making it unrelated to sinus tachycardia. Therefore, after physical activity is the most fitting condition for the development of sinus tachycardia.