In a 27-year-old, a heart rate of 108–120 beats per minute that varies and later settles down on its own is most commonly sinus tachycardia, especially if it occurs during anxiety, stress, exertion, caffeine intake, dehydration, fever, or lack of sleep. Sinus tachycardia can definitely cause palpitations and a feeling of uneasiness, and it is very common for symptoms to fluctuate—sometimes you may feel nothing even at a pulse of 110–114, while at other times similar rates can feel uncomfortable due to heightened anxiety or body awareness.
Atrial tachycardia or other pathological arrhythmias usually present with a sudden onset and sudden termination, a very regular and sustained fast heart rate, often >130–150 bpm, and are more likely to be associated with persistent symptoms such as marked dizziness, chest pain, or breathlessness. The fact that your pulse gradually varies, settles within a couple of hours, and is sometimes asymptomatic strongly favors sinus tachycardia rather than atrial tachycardia.
To be certain, a 12-lead ECG during symptoms or Holter monitoring can definitively differentiate between sinus and atrial tachycardia. If episodes are infrequent, self-limiting, and triggered by stress or lifestyle factors, they are usually benign.
Health Tips
seek medical evaluation urgently if you ever develop syncope, severe chest pain, persistent heart rate >130 bpm at rest, or symptoms lasting several hours repeatedly.
Good luck!