I've had patients who would sleep though a fire alarm prior to their head injuries, but who now wake up when a cat walks by. Sleep is very important to the healing process. If you don't sleep, you're going to be tired throughout the day. If you're tired throughout the day, your memory will get worse and you'll be more cranky and irritable. Lack of sleep makes the other head injury symptoms much worse.
Sleep also has an important role in physical healing.
People may sustain muscle strain as well as head injuries. Muscles may be pulled due to the violent nature of a car accident. If you didn't do that, you would act out your dreams. The brain is going full blast when you dream. When you were young and your brain was not fully developed, you may have done things like sleepwalking.