Not sure if this is a sleep disorder or some other problem, or perhaps it's normal. But sometimes as I'm drifting off to sleep, I'll see a flash of light. It's as if someone is taking a picture with a flash however my eyes are closed.
I thought I've read somewhere that this is similar to your body jerking as you fall asleep, except, it's a visual sleep start.
I've had the light flashes too. Also, a few times (different nights) a loud noise like the static from a TV set when the cable signal goes "off-station", then snaps back in. Sorta shoots across the inside of my head from one side to the other, all within a 1/4 second or so. Kind of weird.