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?

Waiting to hear back from her on the "Log out after xx minutes of activity" setting hidden away in System Preferences, most likely culprit, moved around depending on which OS version.

Maybe it's shutting down for some reason then restarting? If Energy saver is set to Restart after a power failure it would come back up. Maybe the mains loses power, or the power supply or some component in the machine is shutting it off, then it Restarts per Energy Saver settings.

Reset the SMC? -

Shut down the computer.
Unplug the computer's power cord.
Wait fifteen seconds.
Attach the computer's power cord.
Wait five seconds, then press the power button to turn on the computer.

If they set it to never sleep does it do it?

Never heard it put that way. I'll have to remember that one. Geek pickup lines.

Forgot that existed. If in the AM it's at the login screen maybe so, if it's at the desktop that's not it.

Well, it's not the 'logout after xx minutes idle" switch, as that's off.
Console it is, then (that'll tell me if there's been a restart or not), and get your mind out of the iGutter, sir .

If I can't fix it, I'll have to console her .

It's looking like a hardware fault, she's getting it looked at.