I have read once that this happens in a specific mental state. Like, you are not totally asleep, but you are also not totally awake.

Some 10 years ago I had a scary experience: While visiting family in Latin America, one night I was asleep, and at about 4:00 A.M. or so there was an earthquake. Took me a couple of seconds to realize what was happening. Got off the bed fast seeking a safer place. But then the earthquake stopped. Nothing bad happened, nobody got hurt, but it was scary.