As Katrina Schmid, a scientist at the Queensland University of Technology explains, our eyes don't stop working even in the dark, but instead create weak internal signals that mimic light. The colored lights and waves we see are caused by changes in the activity of cells in the back of the eye.
The brain receives these signals and decodes them as random information. The brain does not know that they are not caused by real light, so we are fascinated by colored lights and patterns that are not actually real.

Of course, this condition is also observed in healthy people. But it can also be a sign of retinal problems.

But these illusions can happen even if a person simply presses and rubs their eyes. However, according to Katrina Schmid, it is not recommended to do this on purpose.