I can remember reading once, that the 'Monday morning feeling' stemmed back to the days of working in the 'Cotton mills'.
Somewhere along the article it was stated that, after a weeks work in the 'Mills' you spent your wkd coughing up cotton fibres, and freeing your chest lungs etc of the weekly crap build up. On returning to work on a Monday morning, you were subjected again to the airbound particles which in turn made you feel unwell again, Seems too true for me to make it up, however, will probably do some research to back up my statement, after i have been to MILF etc