One of the signs of a severe head injury is being unusually sleepy, this does not mean you cannot let your child sleep.
You need to get medical attention if:
If they are tired from what’s happened, or from crying, then it is fine to let them sleep. If you are worried in any way about their drowsiness, then you should wake your child an hour after they go to sleep.
Check that they are okay, and that they are responding normally throughout the night.