IMac going to sleep while downloading...

I've setup my iMac so that the screen goes to sleep after 10 minutes and the entire computer after 20 mins. But sometimes I will be downloading something that may take a long time - an hour or so... Do I really need to adjust the computer sleep time everytime I download a big file? Or is there a way around this? the 20 mins sleeptime is fine for normal use. It's just when the computer is actually doing some work by itself I don't want it to go to sleep too soon.
Any help is greatly appreciated...


well what I do is when I know a file is going to take awhile to DL I normal just go to energy saver and adjust the settings. Then the next morning I change it back to whatever.