Jake Gyllenhaal has admitted that taking part in a boot camp to help him get fit for new war movie Jarhead left him exhausted.

"Physically, I was ready for all the running after I killed myself working out, but then mentally it started," he explained. "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 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."

He added, "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."