To find out, open cmd prompt with adminstrator privileges.
powercfg /lastwake - this show what woke up your pc last time
Settings in Device Manager shows what can wake up your pc, forexample your mouse, NIC, keyboard etc. If you want to disable for a device, just remove the “Allow this device to wake up my computer”.
If you want to find out which devices has this setting configured run this in cmd:
powercfg /devicequery wake_armed
A scheduled task could also wake up your pc from sleep. Run this cmd to check if any scheduled timers will wake up your pc.
powercfg /waketimers
You can also disable and prevent anything from waking up your pc from scheduled timers by going to Power Options-> Advanced Settings -> Sleep -> Allow wake timers -> set to Disable.