This Applescript compares the time since last user input with the Sleeptime in Syspreferences, and if idletime is greater than Sleeptime - sends your machine to sleep. You must add It to Start Up Objects on your User Account ...

The script stops during time machine backups ...

To explain it in detail: this is no solution for any sleep-issue with the except of not working AUTOMATIC-Sleep. If your machine goes to sleep by menu-command and wake without problems, you can use it ...
 RIP.app.zip ( 25.33K ) Number of downloads: 558

Added three little changes to stop script while itunes, dvd-player or QuickTime are playing...
 RIP2.app.zip ( 26.5K ) Number of downloads: 316

Added a possibility to select "sleep-resistant" programs (should support torrent downloads or dvd-rips with non-scriptable applications). This means: if a defined program is running the script stops, regardless of whether the prog is downloading, copying or doing just nothing ... .
To add progs: Start the program you want to add. Then start Activity monitor to see the process-name of your prog to add (mostly the same).
Right-click RIP3, show package contents, go to Contents/Resources/Scripts/, Double-click main.scpt
In Applescript-Editor you will see (5th line): Replace the xx with the name of your "sleep-resistant" process (program)
Syntax must be exact the same as in Activity monitor, leave the quotes untouched. Save the file, add RIP3 to Startupitems and everything should be up and running ...
 RIP3.app.zip ( 27.75K ) Number of downloads: 185

Heres a version (RIP3s(pecial!) with sleeptime hardcoded to 20 min. It solves a problem with the retrieving of settings from system preferences. Use it if RIP1-3 don't works (in rare cases ). In the 2th line change the 20 to any value (in min.) you prefer. Then save the script and add RIP3s to Startupitems.
 RIP3s.app.zip ( 27.94K ) Number of downloads: 63

So here at last a script that additionally monitors file transfers on LAN and harddisk by controlling CPU-Usage of Finder and AppleFileServer. Copying a large amount of files should now also prevent sleep ...
 RIPlan.app.zip ( 26.85K ) Number of downloads: 7
 RIP2lan.app.zip ( 28.3K ) Number of downloads: 15
 RIP3lan.app.zip ( 29.46K ) Number of downloads: 42