The Goa'uld palace on P4X-347 had been deserted for hundreds of years. It contains a pedestal which projects a light-matrix hologram onto the ceiling and a hidden device which emits undetectable energy or radiation. The color and light of the hologram interacts with the optic nerve and triggers chemical responses in the brain. Accelerated neural activity causes dopamine-like effects and euphoria. When exposure ends, neural activity decreases, resulting in depression. Addiction is almost instantaneous, with intensity proportional to the length of exposure.
The building was probably designed as a pleasure palace for the Goa'uld. A symbiote can keep a host's mind chemically balanced after exposure, however humans must reduce exposure incrementally over 2 to 3 weeks to avoid the negative side effects. Both SG-1 and SG-5 succumbed to the effects of withdrawal and depression. Lieutenant Barber of SG-5 committed suicide by running into the stargate vortex, and the rest of his team died later from the physical symptoms of withdrawal. Although Teal'c was unaffected due to the presence of a symbiote, O'Neill and Carter suffered from depression and irritability when removed from the light, and Daniel's condition after returning to Earth deteriorated to the brink of death until he was returned to the planet and revived. SG-1 was able to overcome the effects of the light by remaining on the planet until the exposure could be gradually reduced.
Cross Reference: Lieutenant Barber, Hazmat, Loran, P4X-347
Episode Reference: The Light