Android :: Smoothness Completely Lost On Brightness Change Range

I am trying to resolve following task: smooth change of brightness from 100% to 0%, but can't gain effect of smoothness. Want to emphasize that I am using following approach of brightness change. The recommended one.
WindowManager.LayoutParams lp = window.getAttributes();
lp.screenBrightness = floatPercent;
// 0 - 1 window.setAttributes(lp);

Well, it obviously works, but not smooth. I will describe logic. I have a thread that changes brightness:
while (isRunning()) { Thread.sleep(sleepTime);
spentTime+=sleepTime;
view.post(new Runnable() {
public void run() {
changeBrightness();
} } );
}

We have duration of brightness change, for example 10 seconds. I calculate next value of floatPercent (see code snippet above) the way, sleepTime should always be less than 50ms. So looks like it had to be smooth. But I always get not smooth transition. It relates specially the range of 0% - 5% of brightness. Smoothness is completely lost on this brightness range. What can you propose?