Hello, People who sleep on their left side or back may inadvertently put pressure on their vagus nerve. The vagus nerve is an important component of the parasympathetic nervous system and is partially responsible for controlling your heart rate. When external pressure is applied to the vagus nerve, it is possible to trigger an errant electrical impulse which travels the length of the nerve. This harmless impulse can cause a temporary awareness of your heartbeat. Also be linked to eating large meals before going to bed, drinking caffeine too late in the day, or exercising heavily. Stress or anxiety also may be the cause palpitations. Avoid caffeine and alcohol. Avoid heavy meal at night. Do yoga and meditation daily. If symptoms not improved please consult your doctor he will examine and treat you accordingly. Hope I have answered your query. Let me know if I can assist you further.