If you are biting your nails while reading or watching TV that shows you might be perfectionist. There are greater chances for people who are anxious, nervous or frequently bored to bite their nails.
People with this compulsive behavior are unable to relax and perform their tasks with normal pace. Such persons are prone to frustration and dissatisfied with what they have achieved.
If you are constantly biting your nails and that behavior affects your daily life then you are probably dealing with a mental disorder that must be treated. But if biting nail happens occasionally than it can help you dispose energy that is not used productively.