One low-tech tool that we still use to assess head injury victims is called the Glasgow Coma Scale, which measures the brain's ability to function. The measurements include how the victim responds to voice and pain. In an unconscious victim, assessing the severity of a potential head injury means having to wake him up. In fact, how easy it is to wake him up is part of the Glasgow Coma Scale.
Until CT scanners became so readily available, people suspected of having sustained concussions were sent home from the ER with family members who'd been instructed to wake the victims every hour or so. If the family was unable to wake the victim, they were instructed to call 911 or bring him back to the ER.
Now that concussion victims can be scanned to rule out the possibility of a more serious brain injury, doctors no longer need to tell family members to wake the victims. Each case is different, and some doctors will still tell family members to wake victims once or twice overnight, but it's generally not needed.
In no case was keeping a head injury victim awake ever the treatment of choice. If a victim lapses into unconsciousness, there's really nothing we can do outside of a proper hospital to keep her awake. If you are unable to wake a victim suspected of having a head injury, call 911 or take her to the emergency department immediately.