Brains, per se, don't require any sort of environment. If your head was somehow teleported into interstellar space it would take tens of milliseconds for the nerve impulses indicating that everything was not all right to be acquired and processed, and your consciousness would probably last for seconds beyond that.

The horror of the Boltzman Brain idea is that the vast majority of the conscious beings that will ever exist will be created in environments that cannot support them, and will promptly go insane and/or die.