The crowds gathered on The Mall, the London thoroughfare in front of Buckingham Palace, and the excitement of his impending wedding watched by the world made him restless, he tells interviewer Alan Titchmarsh for an upcoming ITV1 documentary on his grandmother, Queen Elizabeth II.
"I hadn't slept at all that night because obviously all the crowds were on The Mall," he says in Elizabeth: Queen, Wife, Mother, which airs in the U.K. June 1.
"They were singing and cheering all night long. So, the excitement of that, the nervousness of me, and everyone singing meant I slept for about half an hour, I think."
His biggest fear?
"The hardest thing was trying to walk down the stairs with my spurs on, sideways. I had visions of myself and my brother colliding and crashing down the stairs."
Still, "It was good fun, it was really good fun," he adds.
His lack of shut-eye recalls the experience of his mother, Diana, Princess of Wales, who told biographer Andrew Morton, in Diana: Her True Story (originally published in 1992), "They put me in a bedroom overlooking the Mall, which meant I didn't get any sleep."
'Grandmother First'When it came to ironing out details for the big day, William says his grandmother was a source of wisdom, particularly when it came to the invitations.
He has recalled that his grandmother advised him to tear up the list of people coming if he wasn't happy with it and that they spoke twice a week on the phone about the plans.
As he has gotten older, William says that they have grown closer. "When I was younger and a young boy growing up, I'd say probably Queen first, then grandmother. But now it's definitely a case of grandmother first, and Queen second."
William says he greatly admires how she came to the throne at the young age of "only 25."
"That's really quite something, to be in a man's world and then as a woman to suddenly be thrust into the position she found herself," he says.
And when it comes to the throne, she will be a hard act to follow.
"They're quite hard footsteps to fill," he says. "There's not much wiggle room left for me to try and find my own path, but I will do."