We think dreams are private, and they are. But sleep researchers are able to see that their subject is having a dream. Sensors placed on the head (and perhaps other locations of the body) indicate the kind of brain waves being produced. Dreams have a particular pattern.
The other indication is known as REM sleep. R.E.M. stands for Rapid Eye Movements. Our eyes move behind closed lids as we dream. But our bodies are otherwise mostly paralyzed.
So there we are in the laboratory, exposed as dreamers, but the content of the dream remains our own drama, to disclose or not, as we choose.
More about that next time. . .