Hitting the mirror was part of the scene but it broke and the actor ended up spending a few hours in the emergency room after badly cuting his hand, said the Hollywood Reporter.
Gyllenhaal, who underwent a dramatic weight loss for the role, is back on the set following the accident.
'Nightcrawler' co-stars Rene Russo and Bill Paxton.
Dan Gilroy is directing and wrote the screenplay, which revolves around an L A reporter who stumbles upon the world of crime journalism.