Think back to the last few action movies you've seen. The odds are good that at least one of them featured a character getting knocked out, whether it was a villain, one of his henchmen, or the movie's protagonist. The fight usually involves a quick blow to the head from behind, and the unfortunate individual on the receiving end goes limp and falls to the floor. A few minutes later, the individual wakes up dazed, disoriented, and possibly bruised, but otherwise OK. Is this harmless Hollywood knockout a real phenomenon?
Despite the popularity of this myth, the answer is clearly no. One-punch knockouts are possible, but the unrealistic part is that it's supposedly possible to K.O. someone for several minutes without any long-lasting ill effects.
Hard impacts to the head are always bad news — even if they don't result in a knockout, they can produce traumatic brain injury, broken bones, intracranial hematoma (brain bleed), or coma. If an individual is unconscious for a few seconds, concussion is likely, but they may be able to stand and walk within minutes, and eventually fully recover. If that individual is out cold for several minutes, they won't just wake up, shake it off, and walk away unharmed.
This video from Dr. Sanjay Gupta provides a helpful overview on how concussions happen, and the effects they have on a victim's physiology:
Even fiction writers know that this convenient knockout isn't real. Renowned science fiction author Poul Anderson wrote, “Writers make man out to be more durable than he is. Their heroes get knocked out, awaken after a while as if from a nap, and plunge right back into action. The truth is, a mild concussion is disabling for periods ranging from hours to days, and as for a severe one, the consequences are not pleasant to watch.“
The takeaway from all this: if you or anyone around you is ever knocked out by a blow to the head, take this injury very seriously, and get medical attention immediately if possible. Despite what we often see on TV, 5-minute knockouts are far from harmless.