Head rests in cars are meant to prevent injuries such as whiplash. Whiplash occurs if you are in an accident and your head is forcefully thrown forward and then back again. The head rest should prevent the snapping back of your head past your body. If the head rest is too low, it won’t stop this action and whiplash can result.
The head rest should be moved up or down until it is directly at the center of the back of your head. However, that is not enough. You have to ensure that you’re sitting properly and not too forward in the seat. Studies have shown that women sit more forward in their seat, which means their head has farther to travel back if they are hit from behind. In this situation, the head rest can’t protect against whiplash. To prevent this, sit as far back in the car seat as is comfortable, reducing the amount of space between the back of your head and the head rest.