Cillian Murphy "smashed his head open" in a freak accident in his hotel room while he was filming 'Oppenheimer'.

The 47-year-old actor worked on the film with actress Emily Blunt, who gave her co-star an expensive pillow to help him sleep while he was holed up in a budget hotel during the shoot, but the thoughtful gift led to a nasty injury as Cillian accidentally banged his head on the nightstand while attempting to flip the pillow over.

During an appearance on 'Jimmy Kimmel Live!', Emily explained: "I like to do a start [and a wrap present]. Bookend it ... No knock on the Holiday Inn Express pillows but I felt like what Cillian needed even more than food was sleep on this movie so I got him this very beautiful pillow ... I'm obsessed with these pillows. It's a Hastens pillow ... "

"So it turned out he was really luxuriating in this pillow one night and he woke up in the middle of the night ... and he went to fluff himself back down in the pillow and misjudged where it was ... the cool side. Flipped it. And smashed his head open on the bedside table.

"So he came to work and they had to like glue his head shut."

Cillian - who is nominated for an Oscar for his portrayal of physicist J. Robert Oppenheimer in the movie - went on to explain he didn't seek medical attention after the accident and instead contacted the film's make-up team so they could fix up the wound for the next day's filming.

He added: "I was a bit shocked you know. I was having a great sleep. I have this amazing pillow ... It was a strange bed, strange table and bang ... We were getting up before it was bright like at 3am so I called her [the head of make-up] and just came in [to work] and then she glued my head and covered it all up and you don't even see it in the movie."