Periodically I Can Hear Music Playing When I Am Laying

Periodically I can hear music playing when I am laying flat on my back trying to go to sleep - it is only for a few minutes - it's done and I fall asleep - healthy 65 year old female who exercises several times a day (to music) am normal weight and take vitamins, eat healthy and on no medications- always busy and -very happy in life with husband, 3 children and 9 grand children - could it be something serious or just normal?