How to avoid sleeping in lectures?

It's not that I cannot hold my attention - I find it a lot easier to put in sustained effort during research - literature surveys, practising derivations, and coding. I also find myself actively engaging in 1-1 (or smaller groups) discussions over long periods of time.

During lectures and seminars, it is a completely different story. I invariantly doze off within 15-20 minutes, irrespective of the content, presenter and class size. I despise it, and sincerely want to change it. Any suggestions?

EDIT : All your answers made me go through all the possible reasons for nodding off. In my case, it is most likely a case of lack of interaction (Peter Teoh's answer) since I have made through with long hours of both mental and physical work for consecutive days (hence mostly not a case of sleep deprivation). Thank you!