Paid sick leave is obviously a good thing, but it's not what I'm talking about here. I suppose it might work differently in the countries you mention, but here if I want to take time off as sick leave then it has to be because I'm actually suffering from some kind of medical problem, rather than just being half asleep and gormless.

(OK, I don't actually have to get a doctor's note if it's for just one day, so I could in practice just claim to be ill, or if my line manager was so inclined then I could agree with him to maintain the polite fiction that I'd been ill. But those options are obviously unethical.)

And in any case, if my real problem today is that it's difficult to get back into a mental state capable of work after three weeks of holiday, extending my holiday by two more days would surely make the problem worse, not better! :-)

Paid sick leave is obviously a good thing, but it's not what I'm talking about here.

(OK, I don't actually have to get a doctor's note if it's for just one day, so I could in practice just claim to be ill, or if my line manager was so inclined then I could agree with him to maintain the polite fiction that I'd been ill. But those options are obviously unethical.)

OK, true, yes.

in any case, if my real problem today is that it's difficult to get back into a mental state capable of work after three weeks of holiday, extending my holiday by two more days would surely make the problem worse, not better! :-)

I'm not sure how that would work with a duvet day, though? Isn't that effectively extending your holiday, too?

Or in other words, what's the difference between a sick day and "turn round and go home and back to bed, on the basis that you're likely to do more harm than good if you continue trying to do work" from the point of view of "getting back into a mental state capable of work after three weeks of holiday"?

Or in other words, what's the difference between a sick day and "turn round and go home and back to bed, on the basis that you're likely to do more harm than good if you continue trying to do work" from the point of view of "getting back into a mental state capable of work after three weeks of holiday"?

A fair point! I think in my head there's some difference between coming in to work, trying to do something, and giving up and going home again, and on the other hand just staying at home in the first place. Probably the former has at least some value in getting me back into the rhythm of getting up early and going to work. But it's pretty borderline, so you have a point regardless...

Perhaps I should also note that I've taken sick half-days in the past when I came into work but discovered part-way through the day that I wasn't up to the task - further blurring the distinction.

Essentially, it then comes down to whether you don't feel up to working because of general listlessness or because of, say, a fever.