From W.Heisenberg, "Physics and Beyond", Harper and Row Publ.

"[In May 1925] apart from daily walks and long swims, there was nothing in Helgoland to distract me from my problem and
so I made much swifter progress than I would have done in Goettingen.....
Within a few days more, it had become clear to me what precisely had to take the place of the Bohr-Sommerfeld
quantum conditions.....I become rather excited and I began to make countless errors. As a result it was almost 3 o'clock
in the morning before the final result of my computation lay before me....
I was far too excited to sleep and so as a new day dawned, I made for the southern tip of the island  where I had been longing
to climb a rock jutting out into the sea. I now did so without too much trouble and I waited for the sun to rise."