WASHINGTON (Reuters) - United States Supreme Court Justice Ruth Bader Ginsburg says there was a good reason she nodded off while attending President Barack Obama's State of the Union address last month: "I wasn't 100 percent sober."

Justice Ginsburg, 81, could be seen falling asleep in her seat during Mr Obama's address to a joint session of Congress in the House of Representatives chamber on Jan 20. She explained herself during an event with fellow Justice Antonin Scalia on Thursday night on the campus of George Washington University.

The moderator asked the high court's senior liberal justice about slumbering during one of the big annual events on Washington's political calendar. "The audience for the most part is awake, but they're bobbing up and down all the time. And we sit there as stone-faced, sober judges. But we're not. At least I wasn't 100 per cent sober when we went to the State of the Union," she said, provoking laughter from the audience.

"Well, that's the first intelligent thing you've done," joked Justice Scalia, a leading conservative on the court who makes occasional public appearances with Justice Ginsburg, including attending operas together.

Justice Ginsburg noted that some of the court's justices traditionally dine together before the speech, and said that Justice Anthony Kennedy this year brought a "very fine California wine" called "opus something or other".

"And I vowed this year: just sparkling water, stay away from the wine. But in the end, the dinner was so delicious it needed wine," Justice Ginsburg said.