it's normally based on your own previously learn viewing preferences based on the screen settings you've chosen, features you've turned on, and surrounding lighting conditions which you can reset any time and as often as you want. if it's in power saving mode it'll set low to conserve power usage you can adjust that as well and same with adaptive battery so it optimizes for battery usage when your not using it. Is it your screen time out feature time maybe and not the adaptive brightness control? when it happens again just short pull on you quick settings and manually put it to what's good for you so it can learn. I just turn it off, I can pick my own view choices this time around thank you Samsung. could be different you never know.