Open Event Viewer, go to Windows Logs > System, and look for a Kernel-Power event with Event ID 41 around the time of the restart. Click it and check the 'BugcheckCode' field — if it's 0, that usually points to a battery/power cutoff (Modern Standby drain or thermal shutdown) rather than a driver crash. A non-zero bugcheck code means an actual crash occurred.
Search 'View reliability history' in the Start menu. Look at the timeline for the day/time of the restart — it will show whether it logged as an unexpected shutdown, a Windows Update install, or a specific application/driver failure. This often names the exact culprit.
Run 'powercfg /batteryreport' in an elevated Command Prompt, then open the generated HTML report and look at the 'Usage history' section for overnight periods. If the battery is dropping several percent per hour with the lid closed, Modern Standby isn't idling properly and is a strong candidate.