About five years ago, I asked a co-worker I was friends with why she'd replied to my e-mail at three in the morning: had she been working late?
It turns out that she is a "segmented sleeper". Apparently, segmented sleeping is when a person naturally wakes up in the middle of their sleep cycle: it's analogous to the notion that people ought to be taking naps at two or three in the afternoon.
So, it was normal for her to wake up in the early morning, do some scholarly reading or work on some journal articles (she's a professor), and then go back to sleep after an hour.
Back hundreds of years ago, segmented sleeping was apparently normal: peasants would wake up in the middle of the night, chat with their spouse, stoke the fire, maybe check on the animals -- and then go back to bed. But in modern times segmented sleeping is mis-described as "insomnia".
So, when I had learned about this back then, I had filed that way as "something interesting". But now it appears relevant: about two or three months ago, **I** started doing this.
Usually my sleep habits are pretty poor: even though my body prefers about eight hours of sleep, I'd typically go to bed around midnight (sometimes 1am, if I was disciplined I'd aim for 10am but really not 'til 11am), and get up at 6:30am. However, about twice a month over the last few months I'd wake up -- totally alert -- get up and putter around the house for an hour or so, and then go back to sleep.
The frequency of this behavior seems to be increasing: it happened about a week ago; and two nights ago; and last night.
Two nights ago, I went to bed around midnight; woke up around 3am or 3:30am (I checked the clock, but I don't remember precisely); lay back down and dozed off and on until 6:05am, upon which I got up and checked e-mail until the rest of the household got up at 6:30am.
And last night I woke up, also about 3am, and again dozed off and on until morning.
It does seem to correspond with when I've been getting sufficient sleep: maybe I don't sleep as deeply when I'm not sleep-deprived?
I've also heard that its not uncommon for sleep patterns to change as you age. And I'm comfortably middle-aged.
I don't mind it, though: kind of interesting.
Addendum: 5:02am, Sunday morning, 5/31/15. This is regarding the next sleep-cycle after I wrote the above: woke up, clock said about 3:50am, dozed, woke again about 4:45am, decided to get up for a bit. And here I am.