i don't know why it does, but be careful. it did the same thing to me and i nodded off while driving my car and ran into someone. it seemed like if i was up and doing things i was fine, but once i sat down to work at my computer or drive my car i could barely keep my eyes open. i told my doctor it made me so tired but he didn't seem to care, just kept dishing it out and said i would be on it at least a year. after 3 months i quit the stuff, the day i hit the car at 4 mgs a day. it was the worst withdrawal ever, had i known how bad it would be i would of tapered or went cold turkey off my DOC instead of starting the sub. good luck and like i said be careful, especially if you drive. peace, amed
