I see what you mean, but my understanding (and by understanding I mean that I have not really read too deeply on the yeast cycle);
1) Only more complex sugars are left over Not sure they get lazy and go to sleep as much as they run out of anything that they can eat. I'd assume that they percentage of complex sugars is much lower than simple sugars and if the yeast don't come in contact with these sugars they have nothing to eat and so go dormant as a means of staying alive.
2) fermentation conditions have degraded and they go sleepy Pretty much the same as above.
In either case, they yeast have changed the way the metabolize the sugar and as I've heard or read, this causes them to produce different types of by-products, some of which might cause off-flavors.
I see it as much the same way that Jamil and others preach to keep ferm temps constant, even if they are slightly higher than perfectly desirable. They would prefer to not have the yeast change their metabolism method any more than necessary.
It would seem to me that introducing more food at the time they are in that mode of operation, and at their peak population would be best, because the only side-effect would be to extend the length of time they are in simple sugar mode.
Don't make me start reading up on yeast, dude! I have enough stuff to think about as it is! LOL!