Sleep isn’t the same across the animal kingdom, and there are a few animals, such as the whale family and certain birds, that sleep in an almost magical fashion. It’s called “unihemispheric sleep,” and during that sleep, one eye remains open, and it does so because only half of the brain is asleep at any given time. It sort of alternates, allowing these creatures’ brains to be both conscious enough to survive and unconscious enough to recuperate. Well, according to a study published in Current Biology (via Science Direct), humans pull off a similar trick in specific situations.

The study suggests people can enter a state where half their brain is in a deep sleep while the other remains in a lighter sleep as to be more alert during threatening situations, such as sleeping in a new place for the first time. It’s unclear if one eye actually remains open when we do this, but it’s metaphorically equivalent, so we’re gonna roll with it. It’s also not the only sleep trick of this nature humans can do.

According to the PBS documentary Mysteries of Sleep, humans are capable of “sleeping with one eye open,” both eyes really, in a state known as microsleep. The phenomena happens when the mind is severely sleep-deprived. The brain will shut down sections for waves of sleep that last just seconds at a time while the subject is seemingly conscious. Kind of like when we nod off during a Lord of the Rings marathon.