I think I remember seeing him mention it somewhere online but I failed to find it within a few minutes of searching.
Edit: See also:
http://en.wikipedia.org/wiki/Non-24-hour_sleep-wake_disorder
http://en.wikipedia.org/wiki/Circadian_rhythm_sleep_disorder#Normal_circadian_rhythms
Edit 2: In the August 27 2013 HP: MoR author's note Eliezer says that he has non-24 sleep disorder (but that he recently fixed it with help from MetaMed).
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 88-89. The previous thread has passed 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: