When we are too comfortable we do not need to learn. When pushed too hard, we are not capable of learning. In between is the small zone, different for each of us, in which learning takes place. This theory of the learning zone (or as Vygotsky said, the Zone of Proximal Development), is widely used at my school, and reminds us that in any class we will have kids spread over all three zones. Hopefully, with skill, we can pull our students into the learning zone for more of their time, thus helping them become more effective learners.
As there were no good Creative Commons version of this graphic, I have produced the one below, so please feel free to use it.