Late actor Heath Ledger had an inability to sleep for long hours, says "I Am Heath Ledger" co-director Derik Murray.

"He'd sleep either zero or two hours a night for years," said Murray.

"He would call people up in the middle of the night."

Matt Amato, who co-founded a production company, the Masses, with the late actor, said: "Heath was very private. He didn't even like any of that behind-the-scenes stuff, like DVD extras."

But later this month, "I Am Heath Ledger", a documentary delving into his life, will premiere at the Tribeca Film Festival.

Interspersed with interviews - among them former girlfriend Naomi Watts, musician Ben Harper, directors Ang Lee and Catherine Hardwicke, and actors Djimon Hounsou, Ben Mendelsohn and Emile Hirsch - are countless clips from Ledger's home videos, dating from his teen years in Perth, Australia, and ending just days before his death in 2008.

They all say he was in good spirits after "The Dark Knight", which had been rumoured to have turned Ledger's typical obsessive role preparation into genuine mental illness.

"This is definitely more the truth than anything else that's out there," said Amato.