Basically, your Sim won't get hungry/tired/need the toilet etc anymore...
It's simple! 
Follow these easy steps:
1) Ctr-shift-C
2) testingcheatsenabled true
3) Shift + click on your sim's mailbox
4) Click "Make all happy"
5) Then click again on the mailbox "Make needs static"
6) Your sim's needs won't go down for however long you play the game