IMac sleeping even when set to never sleep

Running 10.7.5, I've got a fairly new iMac (6 months old) 27", set to never sleep, yet when I got in to work this morning it was asleep. How to fix this?

Output from pmset -g in Terminal:

Active Profiles:
AC Power        -1*
Currently in use:
 autorestart    0
 powerbutton    0
 halfdim    1
 panicrestart   157680000
 hibernatefile  /var/vm/sleepimage
 networkoversleep   0
 disksleep  0
 sleep      0 (imposed by 139)
 hibernatemode  0
 ttyskeepawake  1
 displaysleep   15
 womp       1

And the contents of the com.apple.PowerManagement.plist in /Library/Preferences/SystemConfiguration:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>ActivePowerProfiles</key>
    <dict>
        <key>AC Power</key>
        <integer>-1</integer>
    </dict>
    <key>Custom Profile</key>
    <dict>
        <key>AC Power</key>
        <dict>
            <key>Automatic Restart On Power Loss</key>
            <integer>0</integer>
            <key>Disk Sleep Timer</key>
            <integer>0</integer>
            <key>Display Sleep Timer</key>
            <integer>15</integer>
            <key>Display Sleep Uses Dim</key>
            <integer>1</integer>
            <key>Hibernate File</key>
            <string>/var/vm/sleepimage</string>
            <key>Hibernate Mode</key>
            <integer>0</integer>
            <key>PrioritizeNetworkReachabilityOverSleep</key>
            <integer>0</integer>
            <key>RestartAfterKernelPanic</key>
            <integer>157680000</integer>
            <key>Sleep On Power Button</key>
            <false/>
            <key>System Sleep Timer</key>
            <integer>0</integer>
            <key>TTYSPreventSleep</key>
            <integer>1</integer>
            <key>Wake On LAN</key>
            <integer>1</integer>
        </dict>
    </dict>
    <key>LastSleepUUID</key>
    <dict>
        <key>Date</key>
        <date>2012-12-06T04:39:59Z</date>
        <key>UUID</key>
        <string>019EBED9-FE31-4B63-8D7A-99F41B6EB618</string>
    </dict>
</dict>
</plist>

As you can see, it has record of going to sleep this morning at 4:39 AM, yet all settings also show it's set to NOT go to sleep. Any ideas what's going on?