Rather surprisingly, there’s no rule that shadows must be dark.
Around the time I made this drawing from life, that realization began to click. I credit it entirely to the sense of haste I felt while working from the live model. There’s nothing quite like the pressure of fleeting time to spur a higher degree of spontaneity and efficiency—without which I certainly would have overworked this drawing.
I had no time to draw everything—or even much of it. What if I just ignored all the detail I saw in the shadows?

This approach is effective on several levels:
It reduces the amount of work required by 50%, sometimes even more.
It allows for greater focus on the details of interest—the ones that have the most impact.
It increases the dramatic contrast between areas of mystery and areas of clarity (simple vs. complex).
In other words, this kind of strategy makes the art less laborious, more fun to create, and more interesting to look at.
That’s a pretty big deal!
Now, this is a somewhat extreme example. Usually, I do assign a darker tone to shadow masses, as is natural. Doing so tends to create both a stronger illusion of form and luminosity within the subject. Leaving the shadows as light as this, however, evokes a different kind of atmosphere. The subject appears to meld with the surrounding tone. This makes it easier for certain areas of higher contrast—such as her hair—to attract the eye.
When drawing and painting, we have creative options that the camera does not. It’s part of what makes this art form unique. Yet, we often default to certain "rules" that actually don't exist.
--
The subject of value control is a big one—and more fascinating than it might seem on the surface. I’ll dive deeper into it in the future!