Samadhi means when sushupti, dreamless sleep, becomes alert, awake. When you are asleep as far as the body is concerned, you are asleep as far as the mind is concerned, because there is no disturbance of any dream, there is no tension in the body - but beyond the mind, the no-mind is fully alert. He knows that the mind is without any dreams, he sees it, it is without any dreams, he sees it the body is absolutely relaxed. And this seeing, this alertness, continues twenty-four hours. Then sushupti becomes samadhi.