Do you learn better when you are in comfortable clothes?

Absolutely! This why a child should not go to school hungry. Extremes in temperature and humidity will also affect a student.

Think of when you are at work. If you are overly hot, cold, hungry, your head hurts, your butt is aching from a hard chair, you stayed up too late and just want a nap... Do you have a harder time concentrating? It is the same with kids. A proper working environment, and being prepped for the day, will help them concentrate.