What are the "Sleep States" ?

The target device of the wake up signal must be in a defined "sleep state", more precisely in a "sleep state" S3, S4 or S5.
The ACPI (Advanced Configuration and Power Interface) defines 6 sleep states.

S0 - Power On / Working (switched on, normal operation)

S1 - Processor and system buses are stopped. The RAM continues to be supplied with power and refreshed. - Wake-up within approx. 2 seconds

S2 - Like S1, but the processor and possibly some system buses are switched off. The RAM is still supplied with power and refreshed. - Wake-up takes >2 seconds.

S3 - Like S2, but additionally some chips on the mainboard or plug-in cards (e.g. graphics card) are switched off. The RAM continues to be supplied with power and refreshed. - This mode is often called "Standby" or "Suspend to RAM". - Wake up: >2 seconds.

S4 - The system is switched off almost completely. The information contained in the working memory (RAM) is written to the hard disk (under Windows: C:\hiberfil.sys) and read from there again when the system is switched on. Only the standby circuit of the power supply / mainboard remains active. (e.g. for Wake On LAN) - This state is called "hibernation". Wake up: Significantly over 10 seconds.

S5 - Soft Off - The computer is off. (Shutdown) Only the standby circuit of the power supply / mainboard remains active. (e.g. for Wake On LAN)
Neben diesen Sleep States gibt es noch den Zustand, dass der PC „stromlos“ ist, d.h. die Stromversorgung ist komplett unterbrochen (Schalter am Netzteil bzw. Stecker aus der Dose gezogen.) In diesem Zustand läuft nur noch die über eine Lithiumbatterie versorgte Echtzeituhr (RTC, Real Time Clock) des Computers weiter.