System Preferences/Energy Saver/Sleep, and uncheck Put the hard disk(s) to sleep when possible. If you do not, a program that does not autosave its documents needs that you save them frequently (eg every five or ten minutes) so that the iMac does not consider its HDD to be idle, and therefore to spin it down.
You haven't mentioned your OS, but the above is true for OS 10.2.x, 10.3.x and 10.4.x. However, you mention SimpleText. Are you working in OS 9? If so, the settings are not as obvious, and you should look here and here.
de
Edit: Fixed the links! ~tl
You haven't mentioned your OS, but the above is true for OS 10.2.x, 10.3.x and 10.4.x. However, you mention SimpleText. Are you working in OS 9? If so, the settings are not as obvious, and you should look here and here.
de
Edit: Fixed the links! ~tl