When does former President Obama tend to do his best writing?

In the New York Times article, “Obama, the Best-Selling Author, on Reading, Writing and Radical Empathy“, former President Obama shares that he tends to do his best writing between 10 p.m. and 2 a.m. He states:

I find that the world narrows, and that is good for my imagination. It’s almost as if there is a darkness all around and there’s a metaphorical beam of light down on the desk, onto the page.