This was a real challenge for me in meetings where I wasn't an active participant. Prior to that role I was either teaching or writing reports and so I always had things to do. When I got a job that involved attending meetings I was surprised at how easy it was for me to start drifting even though the meetings were not at all boring. If you're nodding off during a meeting taking notes might help to keep you focused if you're in an environment where doing that is acceptable.