It can be hard to read a fish’s mood or behavior, especially for a first-time fish keeper. They don’t make very many facial expressions like humans do, nor do they have the expressive vocalizations of dogs and cats. They have their own methods of communicating with one another, but these don’t translate well with people. The end result is that it can be hard for a beginner aquarist to tell what their fish is thinking or doing, or even if it’s sleeping, sick, or dead. Once you’ve gained a bit of experience, though, the distinctions between all of these behaviors become easier to tease out. In this article, you’ll learn more about how and when fish sleep, and of course, we’ll also answer the all-important question: Do fish sleep with their eyes open?