I know that when I was knocked unconscious by a falling object at work the ER physician who treated me advised that I should try to avoid activities that might result in head trauma for at least 30 days. He did not say that I would be more prone to unconsiousness, but that repeated trauma would increase the risk of permanent damage.
The Federal Aviation Agency seems to think that one blow to the head that leaves you unconcious for a number of hours (I forget how many) leaves you vulnerable to some future problems. I had a motorcycle accident when I was nineteen that knocked me out for a couple of days. There was a box to check on the form you fill out for your medical certificate (which you have to have for your flying license to be valid) asking if the applicant had been unconcious at any time in the past for (X?) hours or more. I always checked it, and for several years nothing happened. Then one day my medical was rejected, without explanation, and after some months of enquiring and complaining, I got a note from the FAA telling me that the reason was the head injury, although various EEGs had shown no lasting damage. Took me a long time, and a lot of aggravation, to get my medical back. I also know of two other pilots who had the same thing happen. I never did get a good explanation as to just what aftereffects the FAA expected would cause problems.
Only personal anecdote, not data, but here goes. I passed out in class in college once. It was an anthro class discussing a particularly disturbing cultural practice. I was rushed to the E.R., and later examined by a neurologist, with a full EEG and everything. He said I had a seizure disorder and had a 50% chance of having another blackout if I didn’t take medication. I got a second opinion and a second EEG–same diagnosis. This doc said 100% chance of reoccurrence. I took the medicine for a few weeks but it made me feel horrible so I stopped. Also, my Dad mentioned this to his barber, who said that he has a seizure disorder, but that because he doesn’t have health insurance, he can’t afford to take meds all the time. So he just carries the pills around with him and takes one when he feels a seizure coming on. He said he’s been doing it for years with no problem. Based on the expert medical advice of my dad’s barber, I decided to follow this path. 6 years later, never seized or blacked out again. Knock on wood.