A juror in a sex trial has been dismissed from his role after he fell asleep at least twice while listening to evidence.

Judge Michael Crosbie first stopped the trial at the Dunedin District Court abruptly early yesterday afternoon.

"One of the staff informed me you haven't been awake for a while,'' he told the elderly juror, who was still coming around after his catnap.

The jury was asked to leave the room for discussions with counsel and the sleepy juror was summoned.

When the hearing resumed, the man could be heard telling other jurors: "How embarrassing''.

Judge Crosbie explained what had happened.

"We had to have a chat with one of the jurors and he did nod off,'' he said.

The drowsy member would remain on the jury, the judge told them.

When the man drifted off, the court had been midway through an evidential video interview featuring a specialist police interviewer discussing the specifics of sexual allegations with a teenage complainant.

Because no-one was sure when the juror had dozed off, the DVD had to be rewound 15 minutes to ensure nothing was missed.

Later, during cross-examination of the main witness, the man was nudged awake by his neighbour and the issue of his consciousness was raised again at the end of the day.

Judge Crosbie discharged the juror and then told his fellow panel in his absence he was concerned they were becoming distracted having to ensure the man was awake.

"What I did rules out any risk he did nod off. I think he did, only briefly,'' the judge said.

"I had no doubt whatsoever he was trying really hard to carry out his function as a juror.''

Most important, he said, was that the defendant received a fair trial.

"These things happen,'' he said.

The trial would continue with a jury of 11.