Interesting. When I do not feel up to the task of working, whether it is a physical, mental, emotional, spiritual, or arbitrary cause, I use one of my provided PTO days and email the team a short "I will not be showing up to work today" message, without explaining the cause.
I similarly don't bat an eye when a coworker takes off for whatever reason. We're allotted PTO. Why jump through hoops to convince ourselves that it's ok to use it?