"Jake flipped out," a Nightcrawler source tells PEOPLE. "His character was looking into a mirror during the scene and punched the mirror in anger and broke it, cutting himself so bad he had to be taken to a hospital."
And while a source close to Gyllenhaal, 32, confirms the actor received stitches in the emergency room, the source says the reason for the incident was nothing more than a moment in character during an intense scene after an 18-hour day.
"The scene was emotionally charged and his character was talking into a mirror and he got so into it, he banged his hands against the mirror and it broke and cut him," the source close to Gyllenhaal says. "It was all the scene and not because he was mad about anything else."
Another source says the traditionally buff star has also been "intensely focused" on losing approximately 20 lbs. for his role as a crime-scene reporter in the upcoming drama.
Regardless of the reason for the hospital visit, Gyllenhaal returned to set later that day.