Why does my MacBook's monitor go to maximum brightness when waking up from sleep? I have my MacBook connected to an external monitor (via HDMI), and the brightness of MacBook monitor is set to 50% of max brightness . I usually put my display to sleep when I leave my computer for a break, and everytime I wake it up, the MacBook monitor will be set to maximum again. 

Any idea how do I fix it so that when I wake up the display, I do not need to adjust the screen brightness down again?

I am running Lion on an early 2009 MacBook.
Try to reset the System Management Controller.

Resetting the SMC on Mac portables with a battery you can remove

Shut down the computer.
Disconnect the MagSafe power adapter from the computer, if it's connected.
Remove the battery.
Press and hold the power button for 5 seconds.
Release the power button.
Reconnect the battery and MagSafe power adapter.
Press the power button to turn on the computer.