Perhaps more sleep is in order

I get that I'm tired. I do. But this falling-asleep-on-the-sofa thing is driving me batty. Why can't I stay awake just a little it longer, and get ready for bed in a proper and timely fashion? Why do I find myself crashed out on the sofa or the bed, having just woken up from a nap?
Of course, part of the problem is that I lay down, get comfortable, and close my eyes.
Also, I enjoy sleeping, so it's not exactly an activity that I fight against when the mood strikes me.
But waking up after being asleep for 45 minutes, right when I actually should be going to sleep, is exhausting. It messes with my nighttime agenda, as well as my sleepy head.