Screen brightness is an attribute that is reset automatically upon rebooting your machine on Ubuntu and all its fellow distributions. Lucky you if you find the settings okay, but on notebooks and similar machines, you might want to lower the brightness, you it doesn't suck your batteries dry.
To do that, open the following file as root via terminal:
sudo gedit /etc/rc.local
In there, enter the following line somewhere before the very last one that says "exit 0":
echo 0 > /sys/class/backlight/acpi_video0/brightness
0 is the lowest brightness setting. You can adjust it to your likings!