Jet lag?

No, not really. Dozing off during lectures is not unusual, even if you are in your own timezone and have had all the sleep you need. :)
I have this bad habit of waking early in the morning (as I have described before), and now I know that it is light-dependent. Here I wake up as I do at home, when it gets light outside, which is currently around 6:30 local time. Then I get tired when it gets dark, unless something keeps me active (e.g. dancing), but after a 14 hour day of scientific talks, I rather go directly to bed than try to stay awake.

Illustration courtesy: The Economist - Beat the jet lag