How come a computer came out of sleep mode by itself?

This could puzzling when you step away after putting a machine (computer) in sleep mode and then find it is awake while you return. If you are curious enough to find the root cause explaining why the machine woke up while you were not around, use below command in command prompt:

powercfg -lastwake

Note: Option lastwake is available on server OS while on workstations it is not available by default.