Dark viewports

Hi,

I have a model with a main direct target light with several secondary spots 
simulating indirect lighting.  I don't know why the viewports turned dark 
suddenly. The only thing I do was animate the direct light. The problem is 
related to the direct light. The secondary lights seems to work. This is not 
a trouble to the final renders. But I can't see the light effect in the 
viewports with smooth and highlights.