After a certain point, Windows on Bootcamp shuts down on its own

[levmc]

If I leave the lid open for too long and I try to wake it up, I find that if I try to wake it up it doesn't wake up, so when I press the power button, it boots up (implying that it must have turned off). But when it boots up, it boots up right back to where I left off, instantaneously. (suggesting that it actually just waked up)

Does this mean that it was sleeping or turned off?

 

[AndyMacAndMic]

If I leave the lid open for too long and I try to wake it up, I find that if I try to wake it up it doesn't wake up, so when I press the power button, it boots up (implying that it must have turned off). But when it boots up, it boots up right back to where I left off instantaneously (implying that it actually just waked up)

Does this mean that it was sleeping or turned off?

There are different forms of 'sleep' in Windows. There is hibernation and there is sleep. How to configure those:

https://www.howtogeek.com/102897/whats-the-difference-between-sleep-and-hibernate-in-windows/

In your example Windows was probably in hibernation.