Who among us have called in sick to the day job just to put in a solid block of daytime hours on a side project? It's happened to me a few times over the years, and I've been 97% guilt-free about it.
I do that regularly, especially when I'm particularly excited about something new. I've woken up in the middle of the night, registered a new domain, and then emailed the office to tell them I'm not coming in that day. More than a few times.
You do what is best for you.