Time Machine will not do automatic backups
I am running Snow Leopard on Mac Book Pro laptop. I have external HD connected via USB that powers to an outlet. I have Time Machine turned on and directed to use the external HD. However, the only way it performs a back up is if I tell it to Back Up Now or go to Select Disk and choose the external HD. If I let the computer just sit, the automatic back ups do not occur. I do not get any messages that the last back up was delayed. It just indicates when the last back up was and when the next one will occur, though it does NOT occur unless I do the steps above. It simply skips the back up and then states a new time for the next back up, which, of course, does not occur. Is this happening because the computer goes to sleep? Doesn't Time Machine know to wake it to preform the back up? It seems odd that I would have to keep my computer awake all the time just so it can preform automatic back ups.
MacBook Pro