My heart rate is slow when i lay down?is it normal?

im 20 male, not really active in sports, i smoke 6-8 sticks a day. i drink occasionally.when i lay down on bed my heartbeat is 55-57 beats per minute. when i sit its 62-65 beats per minute. and when i stand up its 70-80 beats per minute. i have an echocardiogram a month ago and the result is normal with no pericardial abnormality. the doppler findings is normal too. my blood pressure is always 90/70-100/80. chest xray normal, complete blood count also normal. is it normal that your heart beat is slow when you lay down?