The 'Prisoners' star has been following a strict diet - created by former Men's Fitness magazine editor-in-chief David Zinczenko - and it has left him "ripped" but exhausted.
He told Manhattan magazine: "I'm just off the 8-Hour Diet from Dave Zinczenko of Men's Fitness.
"Water dehydration got me that ripped look. I'm so low energy I'm literally sleeping all the time because I'm on a zero-carbs, limited-calories diet."
The 44-year-old actor's fatigue wasn't helped by the fact Hugh - who is married to Deborra-Lee Furness - can't stay still in his sleep, sometimes confusing fellow passengers during flights.
He added: "I'm the busiest sleeper on the planet. My wife says about every 15 seconds I kick, run, turn around, like I'm running a marathon.
"If you're on a plane next to me, you wonder what's going on with that guy."
The star's regime is nothing compared to some of his Hollywood friends - he previously revealed Dwayne 'The Rock' Johnson gave him "insane" advice when he was trying to bulk up for 'The Wolverine' after finishing filming 'Les Miserables'.
He had said: "I called my buddy The Rock for tips. I lost weight doing 'Les Mis'.
"He said, 'Here's the deal - three hours in the gym and 6,000 calories, eating very lean. You gotta do that for six months.'
"I did it and let me tell you, The Rock is crazy! He is insane. But we did it and the results are there."