Very much as a drug addict would react to the absence of his particular drug. The dwindling or absence of supply is a trauma and the narcissist experiences post-traumatic stress.
The narcissist constantly consumes (really, preys upon) adoration, admiration, approval, applause, attention and other forms of Narcissistic Supply. When lacking or deficient, a Narcissistic Deficiency Dysphoria sets in. The narcissist then appears to be depressed, his movements slow down, his sleep patterns are disordered (he either sleeps too much or becomes insomniac), his eating patterns change (he gorges on food or is avoids it altogether).