Michael,

There are a lot of definitions of a cluster headache shadow...  Near as I can tell the original term came from Bob Kipple, father of the Kip Scale of cluster headache pain.

If you read the Kip scale tab at the left side of this page, you'll see that shadows are a minor cluster headache that intensifies from Kip-1 up to Kip-3.

The simple answer is a shadow is still a cluster headache even if there's relatively little or no pain at all.  As such, a shadow is not normal and should be treated with an abortive at the soonest...  Some shadows fade after a few minutes while others can hang around for hours.  A third category of shadows start benign then ramp up into a screaming a head-banger.

Prior to starting the anti-inflammatory regimen with 10,000 IU/day vitamin D3 that keeps me and hundreds of other CH'ers free of cluster headaches, I opted for oxygen therapy at flow rates that support hyperventilation to dispatch/abort shadows as fast as possible.

Hope this helps.

Take care,

V/R, Batch