Discussing ear fatigue leads me to the topic of low-level monitoring. What most people don't realize is that everything sounds good at paint-melting volume. It imparts such power and drama and your senses are so overwhelmed by sonic mayhem that you don't get a sense of what the track really sounds like.
Since not everyone will be blasting the track through fusion-powered gigawatt-rated speakers, you need to also know what things will sound like at human volume levels. This is where low-level monitoring comes in in the studio.
It actually takes quite a bit of discipline to keep the volume levels low while mixing, because it sounds so much more exciting LOUD! However, the louder you mix, the more quickly you will not only lose perspective, but your ears will wear out, and then you'll start doing things like turning up the cymbals…