The 31-year-old hunk famously bulked up to play a Marine in 2005 movie Jarhead and he showed off his toned physique again in the 2010 blockbuster Prince of Persia: The Sands of Time.
However, Gyllenhaal has now quit his favourite work-out activities - running and biking - because he doesn't want to be in shape for upcoming thriller An Enemy, in which he plays a history teacher who hunts down his look-alike after spotting him in a film.
He tells U.S. magazine Details, "I haven't cycled in a long time. I don't run anymore. Do I take care of my body and take conditioning seriously? Yes. But exercising regularly doesn't fit the energy of the character I'm playing now."
Gyllenhaal admits the laid-back approach is in stark contrast to the training he underwent for his forthcoming police movie End Of Watch, which required him to spend five months preparing for the exhausting 22-day shoot.
He recalls, "Three nights a week in ride-alongs with cops. Fight training every morning at a Kenbo Karate dojo, and I got the s**t kicked out of me. Then (we went to) the shooting range, shooting past each other's heads, with live ammo."