Honestly, as an employee, I don't really want to always hear a reason. Because it does start to sound like an excuse. Just let me know that you'll be out a bit more, and when I can find you. If we have a friendship that includes asking each other about health, then fine. If not, then I don't really want to know.
Also, if someone was constantly saying, "I have to leave early, I didn't sleep last night" it would start to irritate me. Knowing it was temporary would help, but I don't get to leave early just because I'm tired. I don't get to leave early because I ache from head to toe.