I have a general sinking feeling while i sit or lay down.?

often while i lay in my bed and read, or while i sit on the couch and watch tv i get this general sinking feeling throughout my whole body. it literally feels like im falling slowly but steadily. it lasts for minutes at a time. maybe 5-10 minutes. it worries me and i cant find any info on it. im 23, male, pretty sure im in good health. no history of medical problems. anyone know what this is? its been happening for a couple years now about 3-4 times a week. while its happening i still have function in my limbs and stuff i just feel like im sinking and get slightly lightheaded. thanks in advance!