Enabling S3 sleep state in the BIOS
Some BIOSes require S3 sleep state to be enabled or configured within the BIOS before the operating system can take advantage of it. Many modern motherboards do not require (or even contain settings for) this configuration. Your best bet is to check your BIOS' settings, looking in particular at the Power section. Key words to look for are "S3," "Sleep," "Wakeup," and "Resume." The image above shows an American Megatrends BIOS and its S3 settings section.