[jansan]

A japanese guy once told me that he worked in a lab and accidentially caused an explosion, not really injuring him but giving him quite a shock. He claimed that when he woke up the next day much of his hair / beard had turned grey. Although he appeared to be an honest guy, I never believed the story.

Is there any way that this could actually have happened the way he told it?

[pgreenwood]

Yes, it can happen due to alpoecia areata which can be stress related. I had a beard hair turn white almost overnight. I am normally clean shaven but a large section of my facial hair fell out at once and almost immediately grow back white. I think it took a few days but seemed to happen overnight. Also alopecia can tend to attack just coloured hair, so if you're salt and pepper and lose all the coloured you can appear to go white overnight.