Disable keep screen on
I think this should do it:
getWindow().clearFlags(android.view.WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
See API for details.
If you instead set a flag android:keepScreenOn="true" (documentation) only on the views that need to keep the screen on, you wouldn't need to reset the flag manually.
Another approach
getWindow().setFlags(this.getWindow().getFlags() & ~WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
Also read this
and you can also set android:keepScreenOn="true" in the root View in xml.