High glycemic index foods - refined carbohydrates and especially sugar - spike blood sugar, causing feelings of rush and anxiety. The body produces insulin to mop up this spike, resulting in a drop in blood sugar below normal levels and a relatively prolonged period of drowsiness.

The ThreeOclockSleepies can be largely avoided by staying away from high glycemic index foods at lunch.

On the other hand, why avoid them?  It is not unknown, after a nice, leisurely lunch and a siesta, to discover that those seemingly intractable problems from the morning have simple solutions after all...  Or, as SleepToWork points out, the ThreeOclockSleepies may then be a perfect time to clear out the drudge work.