Answer: Cosmic Radiation
The first astronauts were real guinea pigs exposed to all sorts of conditions no human had ever encountered before, including microgravity, extreme temperatures, and full radiation exposure outside the protective envelope of the earth's atmosphere , It was the last element, the radiation that caused particularly strange circumstances for astronauts aboard the Apollo missions.
Both Neil Armstrong and Buzz Aldrin, Apollo 1
But what caused the persistent phantom lights in their view? Cosmic Radiation. We're well shielded here on Earth, but in space both the spaceship and the astronauts inside were constantly bombarded by it. Cosmic Radiation is a high-energy radiation form that permeates most materials. However, if a particle hits certain substances, such as an astronaut's water-filled eyeball, some of that energy can be converted to light. The lightning flashes experienced by astronauts were tiny parts of the cosmic background radiation that triggered a tiny light show in their eyeballs.