I remember something similar happening to me about a month or so after i got my concussion. As you say, it was pretty scary, it always like a sudden shock or a rush of adrenaline. The first time it got so bad that i got muscle spasms all over my body and then i started hyperventilating, all for no apparent reason.

Depending on which specialist i talked to, i got diagnosed with PTSD, anxiety disorder and even a very mild form of epilepsy. I was explained that my brain was readapting after my injury and at night it was harder to switch off.

I was on a short treatment with Clonazepam and it did help but what really did the trick for me was making a very slow and gradual transition to a calm,quiet environment before going to bed and not just turning off all of the lights all of a sudden. Meditation also helped out a lot. After a month or so, i was able to go back to sleep normally.

I suggest you discuss it with a neurologist or a psychiatrist.