Jokes about John McCain’s age have become staple fare for US late-night chat show hosts. David Letterman quipped that the senator was old enough to remember when Iraq was known as Mesopotamia, while Conan O’Brien claimed his Secret Service codename was “Enlarged Prostate”.

Appearing on Mr O’Brien’s show last week, the Republican presidential candidate seemed happy to play along with the joke, pretending to nod off when the host said filming had been rescheduled to accommodate his guest’s afternoon nap.