I have this problem too but I don't think Ive ever fainted or ever came close to fainting.
I especially hate when I have to be up early.. Which is everyday. At the moment I work 6am-2:30pm. I will get enough sleep if I go to bed at around 9pm (which usually never happens).
IF my 2nd job calls me in, chances are they are going to work me 3pm-10:30pm. Which means I am lucky to run home, take a quick shower & be in bed by 11pm. If I sleep 11pm-5am I will get 6 hours of sleep which is enough. BUT I cant just falll alseep after working ALL morning/afternoon/evening, I need time to relax / winde down. I'll probably fall asleep by midnight if I am lucky and only get 5 hours of sleep.
Needless to say, there is never enough hours to sleep during the week. I look forward to being able to sleep in on saturday and sunday mornings all week. Which also sometimes gets cut short if Ive made plans to do something.