Animal research shows again that all calories are not created equal.  Protein compenents (amino acids) activate parts of the brain that regulate

… energy balance, wakefulness, and reward; their loss produces narcolepsy and weight gain.

However, sugar does the opposite – it inhibits parts of the brain that stoke metabolism and maintain a wakeful state.  Therefore, a donut is probably the worst thing to eat for the 4pm dwindles!  We know that protein also satisfies longer than sugar does.

Eggs whites are one easy source of protein.  The authors of the study note:

… protein-rich meals have been reported to be more effective at promoting cognitive arousal than carbohydrate-rich meals (Fischer et al., 2002,Spring et al., 1982-1983).