Do you work for a firm where managers think employees really have to work (what is called) “full time”? That forty hours per week (or whatever is considered “full time” in your profession) is really a necessity? Perhaps you are one of those people with that conviction yourself - that in your job it is really not possible to work ‘part time’.
Of course you are wrong: working five out of seven days is really just as arbitrary as six days, or three – or twenty-eight for that matter. Chopping up the total amount of work that needs to done in your firm into blocks that suit our human physiology has nothing to do with the actual work.