So here's what occurs: You've a plan let's suppose, to awaken at 7; be washed and dressed to kill and breakfasted by 8; at your desk, easel or other workspace by 9; work 3 hours; exercise during your lunch; eat a fit salad at your desk; work 4 more hours; come home; eat supper with your mate; work a few more hours in the evening; and then curl up in bed with a great book. But you don't abide by the plan.
Maybe you awaken late at 8, or 9, ornoon! The plan is trashed before you even get moving. Or, perhaps, it takes you not one, but 3 hours to make it to your desk. And then, when you're there, you drop an hour or three reading the paper, net surfing, and making personal calls. Or, perhaps when lunch comes around you don't exercise and rather than a salad eat a mammoth sandwich and then spend the rest of the afternoon feeling slow and don't get much done. Etc.