Apparently you should first browse carefully FDM options before asking newbie questions. Apparently you should read more topics before asking at all. Apparently you could find here other topics with proper answer 2 years ago, that is shortly after this topic was started.
Hint "For Dummies"
: "Prevent computer from stand by while downloading" works for me. Try to search on your own: search.php?keywords=prevent+from+stand+by&terms=all
and look carefully at the posting dates.
This option works even better now. Can't hibernate even when only seeding (uploading) torrents, no download active.