Developer Marcelo Leite has introduced Should I Sleep 1.0 for Mac OS X (10.7.3 or higher). It's available at the Mac App Store for free.

Should I Sleep uses a combination of heuristic sensors (such as face detection) to check if you are actually at the computer before it goes to sleep. You don't need to touch any button nor remember to activate any system or app setting. W

"Having the display dimmed when we're focused on the computer is certainly annoying.," Leite says. "Staying alert to 'press any key' and prevent (or worse, fix) that, is disturbing and counterproductive. That may happen during many activities, such as a) carefully reading a web page, book or documentation, b) following the progress of a software operation, c) doing a presentation, training or demonstration, d) on a conversation, taking notes between intervals."

When the display is about to dim (due to keyboard and mouse inactivity), Should I Sleep will start its sensors. The camera, microphone all other software processing will stay on for about three seconds. After that (and depending on your sensor preferences) with Should I Sleep, sleep will be allowed or prevented.