Fear or anxiety. Panic attack, in particular, can lead to symptoms commonly mistaken for a heart attack.
Stress. Being keyed up or in emotional turmoil for an extended period, or even depression, can cause the heart to skip a beat, race or pound.
Stimulants. Caffeine is a big offender. A variety of drugs can affect the heartbeat. Antihistamines and over-the-counter diet aids are common culprits.
Alcohol. Binge drinking is a fairly common source of palpitations, even in younger people.
Physical Activity. Heavy workouts or competitive activities can cause the heart to race beyond what normally would be expected. The possibility increases when the activity is accompanied by excitement, nervousness, or other emotional reactions.
Thyroid disorders. Heartbeat irregularities are a fairly common feature of an overactive thyroid (hyperthyroidism).
Mitral Valve Prolapse. The mitral valve controls blood flow from the upper chamber to the lower chamber on the left side of the heart. Prolapse occurs when the valve bulges or balloons out of shape. Usually not a serious condition, mitral valve prolapse causes a distinctive murmur (abnormal sounding heartbeat) and can predispose the heart to heartbeat irregularities.