Why do I wake up with my heart racing after hot dream?

Probably 'begnin' The electrical system of the heart sends regular pulses to the heart muscle to signal it to contract. Many things can disturb this natural rhythm eg electrolyte abnormalities (low potassium or magnesium), excessive stimulants eg caffeine, alcohol, nasal decongestants, certain asociated conditions such as mvp (mitral valve prolapse), anxiety, hormonal fluctuations, dreams etc.