Hit them with an open book quiz. Give away prizes to students who ask questions. Schedule more frequent short breaks. In the third row Jeff has been staring at his pencil for the past ten minutes. Even in the first row, Rob, who usually produces a well thought out question almost on queue, seems to be spending too much time checking the clock.
Trainers know from experience that students learn better in the mornings.
Corporations are demanding more efficiency and accountability from their training budgets every year. A big part of that efficiency is the delivery of training activities in full day programs. The short answer is find another way, any other way, to keep students from falling into the dead zone. I like to have a two minute class stretch then get back to work. Almost any training class can have some sort of group assignment or even a short role playing game that helps solidify core concepts, and keeps people moving.