To grind corn in the old-time grist mills, corn was fed through a hole in the runner stone, and then, by means of centrifgugal force, was carried between two great stones, grinding all the way. However, these two great stones never touched. To accomodate the different grain sizes, the miller was able to adjust the gap between the stones by means of a wheel that was connected by a rod to a lever upon which the central shaft rested. The movement of this adjusting wheel moved one of the great grinding stones, the runner stone, from the other, called the bed stone.

By keeping their nose to the grindstone, the miller could detect the smell of granite, which indicated that the two grinding stones were too close together.

So the term "keeping your nose to the grindstone" doesn't really mean working hard or late. It means ARE YOU PAYING ATTENTION?

To what is important and letting go of the minutae that fills our valuable time.
To what is happening at a macro and strategic level and not the small stuff.
To who is leaving, who is coming aboard, and where you stand in the hierarchy.
To your customer's wants, needs and concerns.

If you pay a little more attention to the important things, you'll grind a lot of grain, and keep your grist mill running strong.