When you’re downloading you probably don’t want your computer falling asleep - this idea (this is something that’s been on ‘the list’ for a while too) discusses the possibility of preventing this from happening.
On Windows there is a new configuration option:
to control this. There are two levels of control - firstly you can prevent complete while a download is in process. Secondly you can prevent sleep while you have seeding downloads that meet the ‘first priority’ criteria ( see the wiki for details )