Speakin through a crack in the limestone boulder late Friday, Abraham Poincheval said he had been buoyed by how his performance has "got into people's heads".
The artist made headlines worldwide when the two halves of the rock closed around him on Wednesday at a Paris art museum.
Poincheval, 44, had carved out a hole inside the rock in his own image, just big enough for him to sit up in, with a niche to hold supplies of water, soup and dried meat.
"People seem to be very touched. They come and talk into the crack, read poetry to me, or tell me about their nightmares or their dreams," he said.
"They are not so much talking to me, I think, as to the stone. I am very happy that the stone has got into their heads."
If he survives the ordeal, the performance artist who has previously spent a fortnight sewn-up inside a stuffed bear, will attempt to become a human hen and hatch a dozen eggs by sitting on them for weeks on end.
Lack of sleep rather than claustrophobia is his biggest worry inside the darkness of the rock, he confessed.
Without a watch -- and with only an emergency phone line -- he has no way to tell the time.
"I can sleep but it is very hard. It is very strange, I don't know whether I am sleeping well or not."
Even though he can only move his feet and hands a few inches, "I do not feel oppressed (by the rock), I feel completely at ease, in real connection with it.
To keep a hold on himself, the man dubbed France's most extreme artist has been keeping a diary which he will publish afterwards.