The experiment involved participants utilizing specialized equipment including sensors and earbuds. On September 24, one participant sleeping at home induced lucid dreaming, a state in which you are aware that you are dreaming. It is apparently a trainable skill, although I have only ever personally experienced it a handful of times throughout my life.
https://pubmed.ncbi.nlm.nih.gov/6877973/
This is the original study from 1983, where conscious eye movements during REM sleep were used to communicate to the researchers while sleeping.
Proving for the first time that communication during sleep is possible, 4 decades ago.