Karma, Baby Whisperer does not know jack about breastfeeding, truly. I can't begin to tell you the screeds of misinfornation in her books and on her TV programmes - and this is another example. I am not getting at you but at Discovery Health who should not be continuing to broadcast this stuff...so excuse the slight tone of exasperation
Milk does not differ in quality (whatever this means - but lets say it means 'fat content' as you suggest) according to whether the mother is rested at that particular moment or not. There is no physiological mechanism for this to happen. Fat in the milk is affected by the amount of milk in the breast - see the link to the kellymom page which outlines this quite well.
You could rush around, do Xmas shopping, swim a few lengths of the swimming pool, run round the block, collect toddler from nursery - and your milk would still remain perfectly ok, with fat in it related to the amount of milk in your breast.
Ditto stress. Stress does not affect the ingredients of the milk or the amount of milk. It might affect let down, temporarily.
Nature is not stupid. Nature protects the infant, and his food supply, and does not let the milk or its supply become affected by stress, dodgy diet, long gaps between meals, lack of sleep, extra physical activity.
We know this, not just from common sense, but also from the scientific literature that looks at lactation.