Windows 10 keeps going into sleep mode

It seems to have started happening over the last ten days or so. In my settings, sleep and hibernation are both set to "never," whether I'm running on battery or plugged in, but if I'm just sitting there idle, it goes into sleep mode after only a few minutes. 😕
However, if I have a movie playing, some music going, or I'm watching YouTube... it doesn't go to sleep.