There are also 4 levels of sleep, your mother board must support them to use them and also what you use to wake the computer matters, to get to a S3 sleep mode you cant use usb in most cases. and BTW S4 is hibernate. I had to read the whole ACPI white papers few weeks ago
Sleeping State Definitions
S1 Sleeping State:
The S1 sleeping state is a low wake-up latency sleeping state. In this state, no system context is lost (CPU or chip set) and hardware maintains all system context.
S2 Sleeping State
The S2 sleeping state is a low wake-up latency sleeping state. This state is similar to the S1 sleeping state except the CPU and system cache context is lost (the OS is responsible for maintaining the caches and CPU context). Control starts from the processor’s reset vector after the wake-up event.
S3 Sleeping State:
The S3 sleeping state is a low wake-up latency sleeping state where all system context is lost except system memory. CPU, cache, and chip set context are lost in this state. Hardware maintains memory context and restores some CPU and L2 configuration context. Control starts from the processor’s reset vector after the wake-up event.
S4 Sleeping State:
The S4 sleeping state is the lowest power, longest wake-up latency sleeping state supported by ACPI. In order to reduce power to a minimum, it is assumed that the hardware platform has powered off all devices. Platform context is maintained.