English mathematician John Wallis (1616-1703) had an odd way of passing time:
“I note that on 22 December, 1669, he, when in bed, occupied himself in finding (mentally) the integral part of the square root of 3 × 1040; and several hours afterwards wrote down the result from memory. This fact having attracted notice, two months later he was challenged to extract the square root of a number of fifty-three digits; this he performed mentally, and a month later he dictated the answer, which he had not meantime committed to writing.”
— W.W. Rouse Ball, Mathematical Recreations & Essays, 1892