But yet, a strict rule of "Work hours end at 5pm" can quite easily lead to tardiness. Not to mention the fluctuating nature of human energy; some days you feel like crap, other days you're inspired to work the whole day (and night) long. Certainly there is a middle ground.
I think the way that we can be kind with our present is to listen. Quit working when your body says no, continue when the energy revving, always put physical health before all else, and most importantly, seek to improve on work quality/quantity in the long run.
There seem to be two types of people. Those who are great at going to failure, past all breaking points, and somehow coming out the other side stronger. These people are the geniuses of the world, and by definition, are few and far between.
For the rest of us, we've got to take it sub-maximally. Push close to but never across our breaking points when we can, and back off when we can't. Slowly but surely the benefits accrue, and great work is done.
Be kind to your present, and the future is likely to be kind to you.
Share the PDF with those who are unreasonable with their present!