Doctor notes are used to restore your energy to full, you can use one doctor note for one full regain. You can use them by ether clicking on the ATM and then the nurse, or clicking on the store and then the nurse, once on the nurse page, it will give you an option to restore your energy for a cost of ether 10 extra credits or one doctor note. You can get doctor notes and credits from jobs but are rare. Sometimes Pimd will also hand out some doctor notes, but this is also rare, and last but least, you can buy doctor notes with real life money from the nurse.