Don't forget the human brain takes about 7 to 8 hours off every day to rejiggle itself, to use a scientific term. The brain's architecture is better than having a training stage but it's by no means able to continually learn without stops and starts.
You see this in young puppies (3-6 month old) a lot as well. They get irritable/exhausted after 15-30 minutes of training, and usually dont seem to learn anything at all during the training activity itself. Then they pass out ("nap") for 30 minutes and when they wake up they do the trick/skill perfectly.
