make screen go black for bed

Mine happens to do just that when I turn off all the lights. It dims down to the clock image then immediately turns off. Well the clock is still visble but extremely faint.
 
I too struggled with this a bit. Tap the bottom left of the screen, tap the brightness to turn ambient EQ (mode?) on or off.
 
Go into the Home app, select the Home Hub and go into settings, under display settings there is an option to either show clock or have the screen go completely off in a dark room