I realized that research describing the different stages of sleep all rely on the assumption that we can accurately determine the stage of sleep using an EEG. I wondered if brainwaves are anything more than arbitrary patterns that appear within EEG data and it's due to our tendency to look for patterns in data rather than these being intrinsic phenomena that actually "mean something". For example like inferring personality from facial structure (which while potentially possible doesn't hold value as personality isn't determined by facial structure).
Can we even say with certainty that extrapolations made from these measurements actually determines the presence of real phenomena? For example: That someone is truly asleep rather than their neurons electrical firings matches a pattern that we associate with sleep. As our only way of determining that is using this device?