Imagine your worst night of sleep. Now imagine fasting all day after your worst night of sleep. Now imagine trying to run a marathon while fasting after your worst night of sleep. Think of how tired you would be at midnight following that day, about how little energy you would have. Think about how short tempered you would feel. In terms of fatigue, that's how it feels to be a new mother on a typical day.
There are days when you don't even feel human. Of course, then your baby smiles or does something amazing - which can be as simple as waving their fist in the air - and that makes up for everything and then some.
There are some days though that you just can't cope and it takes everything in you just to make it through the day. On those days I think you just cope by letting other things go, the cleaning, laundry or whatever just to give yourself a break.