Avoid sleep mode, When ever you leave your computer most of them don't shut down after the vista had launched one of the Microsoft's product. Choosing sleep mode is a very bad idea according to Green Computing as in sleep mode your computer will be in the same state when you wake-up the computer, in sleep mode you need to have a continuous power supply to maintain the system restore to last state. So choosing sleep mode is a very bad idea.
Where as Microsoft has even give an another feature called as hibernation. Hibernate, or S4 in ACPI, meanwhile will save the data in physical memory to hard disk drive(HDD), and then power off the computer. In Hibernate mode, a file named hiberfil.sys which has the same file size as the amount of system memory will be created on the local disk. When user wants to use the computer again, the computer will boot up and load back the state at the last hibernation.
So to "Go Green" use hibernation.