Causes of palpitations when lying down include vigorous physical activity, intense emotions and certain medical conditions such as anemia and hypothyroidism, according to PubMed Health. Other causes of palpitations include the use of medications such as asthma inhalers, decongestants and diet pills, reports WebMD.
Strong emotions, such as stress, fear or anxiety, cause racing or pounding feelings in the heart, explains PubMed Health. Some medical conditions cause the heart to beat stronger and faster than normal and can cause premature heartbeats, leading to palpitations. Some of these medical conditions include an overactive thyroid, anemia, fever, dehydration and low blood sugar. Ingesting illegal drugs, alcohol, nicotine or caffeine can also trigger palpitations.