Can't find library for call to deep sleep functions

I am playing around with a esp12e board, and I would like to put it into deep sleep.
I can do this with a call to ESP.deepSleep(30e6); for example, this works without a problem.

But now I want to call other functions like, for example,

But the compiler does not know where to look for them.
Which Library do I need to install?

Best Regards
Noob Mat

Can you link to the Espressif documentation that says this function should be ESP8266 NoneOS SDK (and thus Arduino ESP8226)? I’m only getting a hit on that function for ESP32 chips.

Yep, you are right… ESP32 is not the same as ESP8266, I was confused.