I teach yoga to beginners and intermediates.  Many are quite accomplished after many months of classes.  I've recently introduced a challenging series of poses and today's group seemed to be especially exhausted at the end.  Perhaps it was the long Christmas break and no exercise during it, or perhaps they ate too much and are feeling lethargic, or perhaps the routine is too demanding.  

I think I'll have to revert to the old - less demanding - routine to avoid a wholesale mutiny!!