I've had this experience a few times, usually ripped and meditating, but not always. I've had it a couple of times ( once laid down in a bath, once sitting here at my computer) straight, and the light for me actually seemed to come from behind and slightly above me. A flash that does actually seem to illuminate in front of me, so it's not just internal.

I remember a Met Cop I knew who was sitting on a train and he was blinded by an intense light for a couple of seconds. Plenty of reasons it could have happened on a moving train, but he said it was exceptionally intense and there was no sun that day. It was a long time ago, but I think he said that other's in the carriage saw it too.

MelT