My friend's just got a 2nd-hand intel iMac, and I think she's running 10.6 on it (I'm waiting to hear back).

She told me that if the mac's left asleep for a few hours (like overnight), when she wakes it up, it's closed down her active apps.

Any ideas what might be causing this? I can't think of anything in the OS system prefs that would do that. I'm hoping to get a look at her console over the next couple of days, which should shed some light on things, but is there something obvious in the OS that I've forgotten about that might do this?
If it's brokenless, don't suffix it...