My computer keeps waking from sleep without any obvious reason

I have similar problems on one of my systems.

When the pc boots up, open a command prompt and type powercfg -lastwake to get a clue why the pc booted up most recently.

In my case, it is Windows Update who wants to check if there is a scheduled reboot, even when there isn't.

It does this by utilizing wake timers, which powercfg -lastwake checks.

To disable Wake timers completely, you can go into your power settings, edit the settings for the current powerplan, then go to the advanced settings, open Sleep -> Wake timers, and set this to disable.

This will prevent any wake timer from waking up the pc, but if you set it to use only important wake timers, Windows Update will still wake the pc, so its pretty useless, especially since that seems to happen every single night, regardless if there are updates.

Use PowerShell (needs Administrator):

PS> Get-WinEvent -Providername Microsoft-Windows-Power-Troubleshooter -MaxEvents 5 | Format-List TimeCreated,Message

You'll get a list of reasons why the computer woke up:

TimeCreated : 04/10/2019 01:00:33
Message     : The system has returned from a low power state.

              Sleep Time: ‎2019‎-‎10‎-‎03T16:27:16.796954700Z
              Wake Time: ‎2019‎-‎10‎-‎03T23:00:33.255687300Z

              Wake Source: Timer - Windows will execute 'NT TASK\Microsoft\Windows\UpdateOrchestrator\Universal Orchestrator Start' scheduled task that requested waking the computer.

TimeCreated : 03/10/2019 09:19:33
Message     : The system has returned from a low power state.

              Sleep Time: ‎2019‎-‎10‎-‎02T22:57:06.677577300Z
              Wake Time: ‎2019‎-‎10‎-‎03T07:19:32.331075700Z

              Wake Source: Device -USB Composite Device

// etc...