How can I stop falling asleep during TV/movies? Does anyone else have this problem?

I work 9:30 to 6pm M-F and when I get out I usually eat a little something, but not a lot (because that would make me more sleepy). I try to eat pretty healthily overall, but that doesn’t always work. I don’t exercise as much as I’d like (was an athlete in college but don’t have much time for it now), but I enjoy being active when I can (sometimes bike to work, go on occasional rides around town, hike, etc). I’m about average in size—I’m 5“7’ and about 145–150 lbs (fluctuates). I usually sleep about 7–8 hours each night. I’m not a big TV person, but my guy is a screenwriter so a couple nights a week we usually watch a few shows together. But if it’s after 9pm or so, I have such a hard time staying awake when he and I are watching TV. I can’t remember the last time I’ve made it all the way through a movie while on the couch (this doesn’t happen in theaters). I’ve tried chewing gum, drinking coffee, twiddling my thumbs, etc. But basically, if I’m on the couch and the TV is on, I’m usually zonked within 45 minutes max, no matter what we’re watching. Help!