Drat. For no particular reason it was one of those nights where I was asleep not very much and awake far too much. I ended up finishing my book, but the day is starting with much yawning. 
@Miraz Aw, that's no good! At least finishing your book is a silver lining. Better than just tossing and turning as you're unable to sleep.
@jayeless [toot.cat] I'm very lucky that it actually doesn't matter. I'll nap later today when the urge arrives. Not sure though why recent sleep has been a bit fractured.