[magicalist]

Not sure about strokes, but it used to be believed that a person with a concussion must be kept awake, but it turns out not to make a difference. It's still a fairly common trope in movies and on tv, though.

One source on that: http://www.uamshealth.com/?id=10724

[Retric]

It's useful to detect problems not prevent them. One of the symptoms of interracial pressure, a significant risk with head injury, is drowsiness. But, other symptoms are also fairly easy to detect in a responsive patent. However, in cases of significant head injury this can be directly monitored allowing the patent to sleep.

http://www.nlm.nih.gov/medlineplus/ency/article/000793.htm

PS: From you link "If the person who is injured is awake and holding a conversation, you can let him or her fall asleep as long as they are not developing any other symptoms such as dilated pupils or issues with walking. "