When my friend Hannah had a baby, someone gave her "Go the F—k to Sleep," the bedtime story written by an exasperated New York dad whose toddler was driving him nuts at night.

It's a picture book for grownups, full of swearing and darkly comic thoughts about children. And so Hannah duly read the book to her newborn for weeks before realizing the gift was intended for her.

"They really should make that plainer for sleep-deprived new parents," she sighed. Which is why the recent announcement — that the hit book is to become a major Hollywood film — is such a surprise. It's a great book, but surely it only has one joke, and one that only tired parents will find funny — if they're awake enough to get it.