The 27-year-old star - who has 23-month-old daughter Harlow and three-month-old son Sparrow with Joel - loves stretching out in her sleep, leaving no room for the Good Charlotte rocker.
A source said: "She takes up all the space. Joel often wakes up to find himself pushed to the side of the bed, while Nicole is asleep smack dab in the middle. She turns herself into crazy positions."
However, Joel is more than understanding about his girlfriend's habit and is prepared to do anything to ensure she gets enough rest.
The source added: "He either crashes in the spare room or falls asleep on the couch. He's so understanding!"
Nicole has admitted to being aware of how annoying her nocturnal habits are for the singer.
She wrote on twitter: "Just heard that I've developed a new habit of sleeping diagonally. Poor Joel!"