Well, all I can say is that I've been vegan for 4 years (no dairy or meat) . . . and I'm still alive. lol.
But . . . I've also been very, very ill just this last 8 months . . . soooo . . . don't think it has anything to do with the non-meat-eating part of my life. The doctors have been unable to diagnose me, as of yet.
But, in answer to your question about meat making you tired. I do know (from many different sources) that meat is the most difficult thing for the body to digest. If you eat a lot of meat and don't have a "tip top" shape digestive system, your body will have to focus ALL energy into digesting that meat, which, in turn, will make you tired (but not necessarily sleepy). Like the way you feel after a cardio-workout? Very worn out and "blah". Don't want to do anything but sit on the couch kind of "tired".
You may also be interested in looking up "meat in the colon after autopsies"? It seems to be found that a person, at the age of 65?, has 8 pounds! of undigested meat sitting in their intestines.
Of course, I can't state any of this as a sure FACT. But, it's what I've come to know through about 4 years of research.