A heart beating fast can be caused by a variety of factors. Common reasons include stress or anxiety, physical exercise, caffeine or other stimulants, certain medications, dehydration, fever, and hormonal changes. It may also occur due to heart conditions such as arrhythmias (e.g., atrial fibrillation or tachycardia). Sometimes, an elevated heart rate happens without a clear cause but often relates to how the body's nervous and hormonal systems respond to stimuli or internal imbalances. Specifically, the adrenal glands release adrenaline in response to stress, which speeds up the heart rate to prepare the body for immediate action. Substances like caffeine, nicotine, alcohol, or illegal stimulants can also cause your heart to race. Other factors like low blood sugar, dehydration, fever, or certain heart diseases contribute to a rapid heartbeat as well. If the heart rate goes above 100 beats per minute at rest, it is considered tachycardia, which sometimes requires medical evaluation, especially if associated with symptoms like shortness of breath, chest pain, or fainting.