This is most likely happening due to a hidden power option called System unattended sleep timeout. 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, Observateur d’événements saying: “The system is entering sleep; Sleep Reason: System Idle”). Vous devez enable this option in registry before being able to change it.
Faites un clic droit sur regedit icon, click Run en tant qu’administrateur
Allez dans HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power\PowerSettings\238C9FA8-0AAD-41ED-83F4-97BE242C8F20\7bc4a2f9-d8fc-4469-b07b-33eb785aaca0
Double-cliquez sur on Attributes
Enter number 2.
Allez dans Advanced power settings (cliquez sur Windows button, write power options, cliquez sur Options d’alimentation, in the selected plan cliquez sur the Change plan settings, cliquez sur the Change advanced power settings).
Cliquez sur the Change settings that are currently unavailable
Click Sleep, then System unattended sleep timeout, then change these settings from 2 Minutes to 20 par exemple.