Windows 10 is restarting automatically after entering sleep mode

I am running Windows 10 Professional on my laptop. In System Settings->Choose what the power buttons do, both When I press the power button and When I press the sleep button are set to Sleep for both On battery and Plugged in.

When I press and hold the laptop's power button, after a couple of seconds, my laptop tus off, and presumably enters sleep mode. However, when I come back a few hours later, it is running again, and the login screen is displayed. This has happens every time, and has been happening for a while.

I don't have a specific time for how long it takes to restart itself like this, as I haven't waited around to see. When it restarts, it doesn't give me any information as to why it might have done so -- for example, an alarm / notification / Windows upgrade / reminder, or any of these kind of things. The login screen just looks normal.

What can I do to diagnose this? Thanks.

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