To be your most productive you need to know when to not work.
We all have days where we're tired our just not feeling up to working on things. In the past I've sucked it up and powered through these feelings and tried to work anyway. The end results are usually a non-productive work session that hurt my morale. Getting sucked into the low morale cycle is the worst thing possible for side projects. To keep going you need energy and a positive outlook.
So my new thing is that if I sit down to work and am not excited to work, and have a targetted task to work on I just don't do it. I've found this helps me come back the next day when I am more excited and keeps my morale up.
Today is definitely one of those days. Lack of sleep is not a good thing for software engineers.