your ears get tired very easilly and the cilla hairs in your ear which is how your brain translates the sound waves into pitch/freq and velocity, your brain struggles to decode the infomation accuratly. when mastering frequent breaks are a good idea, 5 mins every hours does a job but if you ca 15 mins per hour is better, if after a couple of hours your in a rut and can not progress then walk away, come back another day as mark said .. the difference is amazing and sometimes rearly obvious. also make notes at the time , just enough to bring yourself up to speed when you go back to it. but listen to the track before you read the notes... try not to let them influence you when you come back else you will end up where you left off.