But phosphenes also occur at rest with our eyes shut. That’s because the visual cortex creates some spontaneous activity all the time, Samaha says. Where the resulting motifs originate is uncertain, but scientists think they are far from arbitrary. Seeing similar images again and again makes the same neurons fire together, strengthening their connections. We see a LOT of vertical and horizontal lines, such as in walls, doorways, tree trunks, and even the horizon. The more the cells responsible for seeing such lines fire, the stronger their links become—and the more likely we are to see those patterns when we’re trying to snooze.