It does have brightness adjustment, but it's an automatic function not a manual setting like an Aimpoint or Eotech. The "sleep mode" is just the lowest setting.
If you're in a very dark setting you can fool the sensor with a piece of masking tape over it if you want it on bright. Personally, I find it a bit too bright in most light conditions and am looking for a small piece of window tint to see if I can bring the brightness down some.