From every company I have talked to, once a week is the optimal shut down. this gives your computer time to do the things it does when it shuts down and yes folks even when they are off computers are working. Finally this down time allows for components to cool down and take a break.
Think about it like a human body, if you keep up all night you get very sleepy right?..of course you can push yourself but, what happens when you do?..you will start making mistakes, you cant think straight, your muscles start to ache, even shake uncontrolled. Sleep for us is a destresser/recharger same for our comps.