IIRC, there was a bizarre Supreme Court decision about this about 40-some years ago. They decided that the condemned person must be awake and must experience his execution. They decided that it was “cruel and unusual punishment” otherwise.
There was no comprehensible logic to this decision. Go figure.
(Note that this seems inconsistent with the modern three-drug lethal injection protocol. I’m not sure if lethal injection was a thing at the time of this decision. It made no sense anyway.)