Williams, who had a daughter with Ledger before their relationship ended, said the "Brokeback Mountain" star always appeared to have unlimited energy that disrupted his sleeping patterns, the Daily Mail reported Saturday.
"For as long as I'd known him, he had bouts with insomnia. He just had too much energy," Williams told the British newspaper.
"His mind was turning, turning, turning -- always turning. He had an uncontrollable energy. He buzzed. He would jump out of bed."
Williams told the Mai Ledger, who was found dead of an apparent overdose in January, typically managed to hide those underlying issues from the outside world.
"You can pick up on it in his performances, but it's easy to overlook because he was so physical and beautiful and strong and masculine," she said. "But there was always that underlying sensitivity.
"That's who he was."