JAKE GYLLENHAAL spent four months getting in shape for gritty new war movie JARHEAD only to find he wasn't mentally fit for the bootcamp training.

The actor tackled triathlon training to get fit for the Sam Mendes film and could run rings around his co-stars in bootcamp but sleep deprivation and barracks antics left him dazed and confused.

He explains, "Physically, I was ready for all the running after I killed myself working out, but then mentally it started. The first night of boot camp I got an hour and a half worth of sleep.

"I woke up in the middle of the night because I later found out (co-star) Lucas Black had thrown a bottle in the air and it hit me on the head when I was sleeping.

"Those things were constantly happening and, by the end of that week, I had gotten cumulatively 10 hours of sleep. My head was in such a weird space."

His mindset led to Gyllenhall making one of the most foolish decisions of his life.

He adds, "I had a 10-and-a-half wide and a 10-and-a-half regular (boot size) and I had loaned one of the guys my 10-and-a-half wide boots.

"I thought my other boots would stretch and it would be fine but I was standing there for two and a half hours and my left shin was hurting so badly."