The single most important thing a teacher must do is manage the learning state of his or her students. Twenty minutes is probably the maximum time that most people can stay in a positive learning state without a change of stimulus. Most students may even be aware that they have moved down the spectrum from a positive learning state to a feeling of the brain being out to lunch!
An effective teacher is aware of the looks on the faces of students, the squirming, and general disinterest. These teachers are always scanning the class for signs of learning fatigue and know what to do to ameliorate the problem.
For middle school, the rule could easily be reduced to the 15-Minute Rule. Teachers cannot expect middle school students to stay in a favorable learning state for more than 15 minutes without intervention. A maximum of 20 minutes for high school students and probably most adult learners, including those in graduate school, is appropriate.
We could learn from observing how lower-elementary teachers handle learning fatigue. Since students in the early grades generally have a limited attention span and suffer from learning fatigue in a more demonstrative way than older students, elementary school teachers are constantly forced to manage the learning state more acutely than teachers of middle school and high school. Visiting a good teacher of second graders, for instance, would provide an opportunity to see a master of state management, since most second graders are able to stay continuously in a positive learning state for only a short time.
Visit college classes, particularly graduate education courses, and you will frequently observe a professor lecturing on and on and on. The concept overriding almost all other objectives is to cover the material. The most expedient way to do this, of course, is to teach didactic lessons. Creative teaching strategies are talked about but, because of limited class time and a great deal of material to cover, are seldom demonstrated fully.
Students at this level know how to play the game and are habituated to the didactic lesson. The eyes make contact, the pencil is poised, the laptop is humming, but after about 20 minutes without a learning-state change, even the brains of these high achievers are on autopilot, with very little learning taking place.
As we see a higher percentage of courses taught online, the challenge to demonstrate the management of learning states becomes even greater. These classes now become the models for teacher preparation.
The most effective teachers know intuitively how to read their students and have almost a built-in clock that reminds them that middle school and high school students need to have their learning states managed about every 20 minutes. Naturally gifted teachers and speakers change the learning states smoothly and without even their own awareness at times.
As you experience an outstanding presentation from a gifted speaker, make mental notes to see how often the speaker changes the learning state. It may be a change of voice, a joke, a gesture, a storytelling, or a relocation of the speaker -- anything that allows the participants to change their state!
In classrooms where teachers are effective learning-state managers, students know what to expect and often look forward to getting out of their seats periodically and interacting with others, ultimately refreshing their learning states. Some teachers say that once they get the students settled into their seats, they want to retain that structure for the remainder of the class period if possible.
The problem isn't as critical when students are involved with cooperative learning, project learning, or other activities that require students to move around and be involved in hands-on learning and discussions with other students. The bottom line is that students need to have their learning states changed frequently.