My body making me sleep??

Patient: I am 29, work part time and have no problems falling asleep. I get home from work at night, eat dinner, and watch some tv or workout before going to bed. I usually get 6-8 hours of sleep every night. the problem is that when I sit down for a little while, I will fall asleep, before going to bed. I’ve been to the doctor for blood tests and they said there is nothing wrong with my metabolism or thyroid. While I find it great that I can fall asleep anytime, I am tired of not being in compete control over myself. I’m not overly tired when I get home, so I’m not sure what is causing me to essentially pass out. I generally have blood pressure from normal to the low-ish side and my oxygen levels tend to run a little low-there have been a few times, that I almost couldn’t donate blood because of either thing. I appreciate your help, thanks.