It must be ...

You can feel it slipping out from underneath you, your sleeping patterns.  Tired, but can't sleep.

It is 11.30 pm and you can't sleep and so you read until you can.  Which in your case is when you put the book down at 1.48 am.

This nocturnal sort of behaviour means it must be getting close to the winter solstice ... oh, look, there it is, four days away.

On the minus side, this whole deal is disruptive.  You can feel it rising up as anxiety in your body, gripping around your heart.  On the plus side, you work from home (this whole deal is one of the reasons why you do) so it's not too disruptive.  On the plus side, you are getting to read a whole lot :)