Some sort of tradeoff
Yesterday I was more tired than usual (I thought maybe I was coming down with a cold), so I went to bed at the remarkably early time of 8:20pm. Usually "10pm" is early; my usual bedtime is more like 11pm.
So, I went to bed -- and woke up at about 1:30am. Or rather, a kid woke me. Fair enough -- except that I couldn't get properly back to sleep. Whereas, usually I'm a fairly robust sleeper: if interrupted, I roll over and immediately fall back asleep.
I drifted in and out -- and then at about 1:45am, I came up with a song idea (both melody and lyrics). After lying there about five minutes, I figured that I ought to record it, rather than lose it -- so I went and got my cell phone, and sang into the voice recorder function. Then I put my phone back, and went back to bed.
And then I came up with another line. Got up; recorded it at 1:48am; back to bed.
Another line; 1:50am; back to bed.
Around this point I just took the phone with me.
1:51:40. Also 1:51:57.
2:09am. Also 2:10am.
All of this was contributing to the same song. So I think I have a complete song now. In fact, I'll probably have to edit it down and drop the least-strong lyrics: that's a strong position to be in.
3:20am, a song concept, in the form of a single line. I jotted it down in the "Notes" app; will develop it some other time.
Got up at 6:30am. So, I got 5 + 3 = 8 hours of sleep. That's enough. I was a little groggy when I first got up, but I'm fine now.
And it's an acceptable trade-off: interrupted sleep, in exchange for a full song plus an additional idea.
As an aside: I'm pretty sure this is called "segmented sleep". It's perfectly natural. It happens to me when my sleep needs are "saturated": when I go to bed late and only get six or seven hours of sleep, it doesn't happen: I fall asleep almost immediately, and sleep straight through.