unplug the network cable and see if the machine stays asleep.
Sometimes network drivers have Wake on lan (WOL) set and another computer attempts to connect to the network card and wakes the machine back up. (other computer on same homegroup, or a media extender, xbox,...)
-to fix you need to config the network driver and not allow it to wake the computer.
also, to confirm:
start cmd.exe as a admin (windows key +x, then type a)
powercfg.exe /lastwake <- this will display what the computer thinks woke it up
Also, note: your computer might be attempting to go to sleep, start to shutdown. The OS will tell all the programs to shutdown
but a program has the option to block the sleep attempt and the system will come back to life (never did actually go to sleep)
common with media players that can stream or provide service to other computers or devices on the same network. You have to configure the
power management to not let them block the sleep request.