whether you are drawing or painting keeping the effect of light is one of the most important elements in a piece. The key to this is to keep a tonal gap between your darkest tone in the light and your lightest tone in the shadow.This gap can be clearly seen it fig.1 where i have isolated the light tones and shadow tones from an oil portrait. there is a clear gap between the two bands of tone representing the light and shadow. so it stands to reason that the more tone you use in your lights the darker your shadows need to be in order to maintain the gap. if there is alot of modeling in the lights this may be necessary but can be over come by the use of temperature instead of value to represent smaller changes.