For most of our history, sleep looked something like unconsciousness because our only means of studying it was to look at a sleeping person and make assumptions based on that.
When you’re asleep, you’re largely non-responsive and for all intents and purposes ‘dead to the world’, but intuitively you more than likely know that is not the case. If someone is in a coma, has passed out, is inebriated or has been sedated/anaesthetised then they can be very difficult to wake up. You can slap them, shout and perform surgery in some cases but nothing happens – sleeping people are comparatively easy to bring back to an awakened state.
Still, we didn’t really have any means of assessing what was going on inside any brain, waking or sleeping, until the 1920’s when Hans Berger, a German clinician, used emerging technology which had been tested on animals to read the electrical activity within the skull of a live human subject. What Berger measured for the first time are the now well understood and commonly spoken of ‘brain waves’ (though what he was looking for was a physiological basis for psychic phenomena).