Funny one. They clearly have the potential to help see the game better but the knock your credibility would take isn't worth it.
Are there any rules/advice to referees on the wearing of sunglasses during a match?
Thanks - that is good advice - and one that I would advise a referee (I'm not one) but are there actually any regulations or formal advice to this effect (stubborn person syndrome....)
Certainly very unusual. Not entirely unheard of, though. Would like to know why you're asking.
(The major disadvantage of sunglasses is you can't make eye contact with anyone; there are some sports that allow them for practical reasons, but they all advise that if you want to have a conversation with a player, you take them off or move them down your nose so you can have that direct eye contact.)
"Words of advice" re the wearing of them would be better if there was something in writing somewhere to that effect, rather than anecdotal.