The worst part of being The Times national su doku champion is the way the puzzles can give you nightmares.

Nina Pell, a 21-year-old mathematics undergraduate at Sheffield University, became this weekend the first person to lift the trophy twice at the championship finals.

In the first round, Ms Pell astounded the organisers by completing four fiendish su dokus in just 18 minutes. She then won the grand final by finishing four even harder puzzles in 31 minutes.

Ms Pell, who first won the £1,000 prize in 2005, said that she no longer devours puzzles in su doku magazines because she began to show symptoms of su doku overload. “I’ll always stop when I start dreaming about numbers,” she said. “When I get weird delirious dreams, that’s when I know it’s gone too far. I’ll probably have a su doku dream tonight – it’s a bit more of a nightmare, really.”

The dream does not involve solving the puzzle, but watching as numbers appear on a grid: “I don’t see myself doing it. It is just solving itself.”

Ms Pell admitted that her victory on Saturday at the Institute of Education in Bloomsbury, Central London, was partly down to luck. At a critical moment during the final, in which eight contestants remained from a starting grid of 100, she made a guess in the knowledge that she had a 50-50 chance. If she had tried to work the puzzle through, it would have added vital minutes to her time.

“It was a four or an eight. I went with a four. If I’d got it wrong, I’d have to have rubbed it all out. It would probably have taken about ten minutes to do it all again.” Her winning margin was two minutes.

In the end it was Miss Pell’s consistency that was her most important strength. Tom Collyer, who won last year, completed the four puzzles on the final paper in less than half an hour, but his results were not admissible because he had been knocked out in the qualifying round. He said that he had made mistakes in his haste to be the first to finish.

Asked whether puzzle-solving remained fun under competitive conditions, Miss Pell said: “No. It was so nerve-racking. When I held up my number in the first round, my hand was shaking so much I thought [the invigilators] wouldn’t be able to read it.”

Howard Granville, who came second, said that the tension in the hall was nearly unbearable. “You’re acutely aware of the sound of paper being turned over, and the steps of the invigilators going to take in someone’s paper. It is horrible. Could the invigilators just wear socks next time?”

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