I've got a pretty regular and predictable usage pattern with my computer - quick logon in the morning to check emails, go to work, use computer at night, restart before bed.
The computer is set to sleep after three hours of inactivity. So in the morning it will be sleeping, then when I get home from work same thing. Sometimes not; never figured what wakes the computer, but 90% of the time, that's how that works - it sleeps after three hours, and stays sleeping for several hours.
However, this week I noticed that the computer seemingly never sleeps. It's always on when I come by it, including in the morning, and when I get home from work.
Checked the system log in the event viewer for events 1 (wakeup) and 42 (sleep), and I noticed that, into the morning of 9/16, the normal pattern of several hours of sleep was adhered to. Since 9/16 though, every sleep period lasts about 2 1/2 minutes. That is, it goes to sleep (event 42), and then within 2-3 minutes, it will wake up (event 1).
There have been no Windows updates in that time that would affect this. Definition updates for Windows Defender on 9/18 and 9/21, and KB2744842 (security update for IE 9 on x64 systems) on 9/22, but nothing happened, including no new software installations, on 9/15 or 9/16.
So why is my computer suddenly waking from sleep almost immediately?