system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why I see patterns and swirls when I'm in complete darkness.

When you're in bed before sleep thinking about stuff and you see swirls and patterns moving around in your vision.
gpt
The cells in your eyes that turn focused light into signals in your brain are always firing randomly. They have a baseline level of activity that you can think of as noise in your visual system. This also happens downstream, in your brain. 

When your eyes are closed, or when you are in a very dark room, there is nothing to drown out that random noise. It is the same thing as tuning an older tv to a dead channel and encountering snow or static. 

This is also why hallucinations are more likely in low-light environments, and why during psychedelic experiences visuals can suddenly pop out when you close your eyes. Your visual system is "quieter" so your brain has nothing to focus on other than "noise". 

Those patterns and swirls are always there, you just have an image of the world distracting you from them when your eyes are open.