N900 does have light sensor. You can check it by just shading upper left corner with your hand - the screen will darken and then after removing your hand it will lighten up.
If you put brightness to 2nd level then it will be 2nd during light condition and 1st when it is dark around.
Are you sure you are not mistaken in your post - it is not natural to need lower brightness in light and higher in dark conditions. It is normally just the opposite and that's what display light sensor does(quite well) .

PS. It is "Power saving" option in Display settings that activates the light sensor. Check if it ON on your device.