It is very hard to understand how debilitating sleep deprivation is until you suffer from it. There is a reason we, as humans, need to sleep. Our bodies need to recharge, our brains need time to process everything they take in, without having to take more stuff in, our muscles need to relax for a little while to make them stronger for the next day, and our eyes need to close, to rehydrate and allow us to see clearly for another day.
When you don’t get enough shut eye, these things can’t happen, or at least can’t happen to their full potential. We’ve all had bad night’s sleep where we feel rough the next day. Now imagine that bad night is every single night. That is when sleep deprivation really takes hold.