The primary adjustment for the brightness should be in the monitor itself. OS X includes a tool to help you adjust the settings, by showing you patterns with instructions about how to adjust the monitor's settings. You are also prompted to adjust the output from your Mac at the appropriate time. If you make adjustments from the wrong place first (by not using the tool) you may get a less pleasant, effective, and more fatiguing image. To use the tool, go to System Preferences > DIsplays > Color > Calibrate.