A tired you performs like an unskilled you.
You’re not being a hero when you work late hours and burn the midnight candle. Study after study shows that sleep deprivation is the worst thing for a mind that wishes to accomplish things. 40 hours is enough, folks. If you can’t get done what you need to get done on a daily basis in eight hours, then you need to rethink your angle of attack. Extra hours only produce more errors that need to be fixed at a later time. Ernst Abbe stated that going from a 9 hour work day to an 8 hour workday actually increases output.
When you’re overworked and under rested you become cranky, irrational and in my case, turn into a monster. Studies have shown that a sleep-deprived brain loses activity in the parietal lobe and the occipital lobe, the very parts of the brain that control visual processing and our understanding of objects and numbers. Unless you’re a stripper or a landscaper (no disrespect to either) this is not good news, I’m sure you can agree.
The part of the brain that does get overworked in our tired selves is the thalamus. The thalamus is responsible for the regulation of consciousness, alertness and sleep. So our brain is spending energy on simply being awake. Even one night of sleep deprivation (and 5 hours is considered sleep deprivation) can have unnerving effects the next day.