To solve this problem, you must reset the so-called SMC memory, after the reset, Mac OS X will add the right settings to SMC.
To reset the SMC memory follow the following steps:
Step 1: Turn off your MacBook Air completely.
Step 2: Connect your MagSafe adapter to your laptop.
Step 3: Hold down the keys shift⇧ + ctrl + option⌥ + Power simultaneously and turn on your MacBook Air.
After you completed these steps correctly, Mac OS X will reset the SMC memory. Your MacBook Air will go into sleep mode after you close the casing. If this is not the case then you may want to complete the steps above again and make sure that you press the right keys on your MacBook Air. The SMC reset also works on MacBook Pros that are suffering from the same problems.
An SMC reset can also help if there are problems with an external display or with the battery.