According to the techniques herein, one or more sensors coupled to an aligner are used to sense one or more physical qualities of the aligner to determine whether the physical qualities indicate that the aligner material has relaxed and has, e.g., reduced the force systems applied to teeth and/or that the teeth movement for the stage has slowed down. The sensors may provide a signal that represents whether or not the physical qualities fall below a threshold value. A treatment plan may be modified if needed. These techniques have the potential to make treatment plans faster and more effective by speeding up some stages while making sure some, perhaps other, stages are implemented only after the teeth are near the appropriate position.