Click Sleep, then System unattended sleep timeout, then change these settings from 2 Minutes to 20 for example.

Click on the Change settings that are currently unavailable.

Go to Advanced power settings (click on Windows button, write power options, click on Power Options, in the selected plan click on the Change plan settings, click on the Change advanced power settings).

Right-click on regedit icon, click Run as administrator.

You have to enable this option in registry before being able to change it.

On my machine it was set to 2 minutes, which was mighty annoying, as it would cause behavior described by OP (sleep, wake-up, login screen, Event Viewer saying: 'The system is entering sleep Sleep Reason: System Idle'). You can configure automatic login for Workflow users only on individual computers. You can configure automatic login for Windows users on individual computers or on a global basis. This is most likely happening due to a hidden power option called System unattended sleep timeout. Go to Advanced power settings (click on Windows button, write power options, click on Power Options, in the selected plan click on the Change plan settings. Automatic login and logout can be configured for individual workstations or for all workstations that are included in a global default configuration.

Windows 10 automatic logout