There seems to be an upswing in interest in in reporting about the effects of bacteria in the digestive system ..probably because probiotic drinking yogurts are currently the fastest growing dairy product in Europe. Inulin is a soluble fiber, found in chicory root, yams, artichokes, onion, garlic, and agave. Added to yogurt, it increases the absorption of calcium, as well as making non-fat yogurts taste smoother.
The Wikipedia entry on Inulin suggests that it is useful for managing blood sugar levels. So yogurt, or other foods containing Inulin, might help to smooth the blood sugar spikes which affect mood and energy levels.
Update - Trader Joes sells Inulin as a supplement (in powder form) and suggests adding it to yogurt.
Further reading - each of these has an obvious bias, given the site publishing it
From a site for dairy food processors.
http://www.dairyfoods.com/CDA/Archives/6fedc4413a0a7010VgnVCM100000f932a...
From a site for decision makers in the supplement and healthy food industry
http://www.nutraingredients-usa.com/news/ng.asp?n=76013-prebiotic-probio...
Article on a wide range of Prebiotics - PDRhealth provides information to health care providers
http://www.pdrhealth.com/drug_info/nmdrugprofiles/nutsupdrugs/pre_0326.s...