I think it feels bad because it inevitably messes up one's digestive clock (which is implicated in robustly configuring the sleep clock) and by implication, particularly with modern food, blood sugar levels.

(I suspect that the long cultural stability of the Spanish long siesta has a lot to do with the cultural stability of mealtimes -- and meal types/sizes -- that developed around it.)

The other day I felt queasy, suddenly, after lunch at a normal time -- so queasy I had to write off the day. I then proceeded to sleep for the best part of 24 hours, because all the phases seemed to merge. A literal 24 hour period, useless. Other people lived normal lives in that period; for me it just vanished.

I think blood sugar had a lot to do with that, as well as the time of year (winter in Britain) and it has definitely unnerved me.

I cope by just accepting it; trying to do things when awake. But there are things you can't really do in the middle of the night if you live in a close, quiet neighbourhood; hoovering, running laundry, shredding documents, even some printers are noisy.

Because it takes time to adapt fully. Your polyphasic periods are more like stopping and restarting a routine that disallow your body from adaptation. Those who adapt fully over the course of a month feel as good as you do during monophasic sleep.