If the sleep mode is greyed out you need to do the following.
1) Update your graphics drivers and chipset drivers.
2) If the above step doesnt solve the problem, restart the computer into the BIOS settings, and go into the Power Management section in the BIOS. Make sure ACPI mode S3 or S1 is enabled. remember, if you have an incompatible video card, then one or more of these states may be unavailable.