I think maybe twice in about 12 years of working nine-to-five (more like 8.30am to 6pm actually) I woke up feeling jaded and unmotivated, and so decided to call in sick when really I was just exhausted.
From memory I justified it to myself by contemplating all the days when I was slightly unwell yet still managed to turn up to work and put in some solid slog. In the scheme of things I considered that my soldiering on mostly seemed to balance out the isolated incidents of lethargy.
The dignified term for this type of spontaneous day off is "having a mental health day" which lends the practice a veneer of respectability that "throwing a sickie" can't muster.
If I recall correctly there were unofficial guidelines covering the execution of a mental health day. Avoid Mondays or Fridays - or the boss might think you're trying to snag a long weekend. You must telephone to report your faux illness before the actual time you were expected in at work. I imagine that today people might text or email, in which case there's no need to practice the croaky, weak voice that reinforces your story.
I'm always amazed at the people who venture out in public when they throw a sickie.