What do you mean "there's no information on what do they actually measure in every individual"? We are measuring slow-waves, the synchronous firing of neurons which defines deep sleep. This is a real-time feedback system, with measured ERPs on a 5-second on, 5-second off stimulation protocol?
Happy to answer any questions, if I'm not understanding your statement correctly.
Happy to answer any questions, if I'm not understanding your statement correctly.