There are usually a few on a tablet or other android device about the screen going dark.
One is "Sleep" which is what you said you set. Another would be to check the settings on the device for how it reacts to different light. Some tablets and phones will change the screen dependent on how dark or light the room you are in is.
There is also the possibility that there is an app on the tablet that is the cause. Some come with "Eco Mode" or something similar, which can change the screen and other options on the phone when it reaches a certain amount of power.