"Scientists just discovered the first brainless animal that sleeps"
The submitted title, "Scientists just discovered the first animal without a brain that sleeps", is ambiguous. In addition to the correct meaning, it could also have meant the first animal whoe brain does not sleep.
Are there animals who don’t sleep but whose brains sleep?
While ambiguous from a purely grammatical point of view, it does not seem ambiguous from a “common sense” point of view.
https://en.wikipedia.org/wiki/Unihemispheric_slow-wave_sleep
Many scientists believe that to solidify and organize memory and learning the brain needs a period with little or no environmental distractions, and that the purpose of sleep is to provide such a period.
Pelagic fish live in an environment with such a sparse environment that their brains may be able to do that processing without disconnecting and so do not need to sleep.
This means it is plausible that there are animals with brains whose brains do not sleep. If such an animal is shown to exist, it will be the first such case, and so a headline saying that the first animal with a brain where the brain does not sleep is a plausible headline.
2. There are a variety of externally observable behaviors of animals that indicate that the sleeping (especially when "externally observable" includes "observable when hooked up to a variety of instruments").
Not all of these are associated with functions that require a brain. Hence, it is plausible that there is a brainless animal that exhibits some of these sleep-associated behaviors, perhaps even enough of them that it would make sense to say they sleep, even though they have no brain.
Hence, a headline saying the first animal that does not have a brain yet sleeps has been found is plausible.
If we have to decide that one of these interpretations is the one people would expect by "common sense", I would argue that it is the first one. Yes, I realize that most people will not know about our uncertainty over the sleep or lack of sleep in pelagic fish, so would not come to the first interpretation by that route.
However, most people's experience with sleep is based on their first hand knowledge from doing it themselves almost every day, and from talking to other people who do it, and from observing it in fairly intelligent animals such as dogs and cats. They are going to think of sleep as a cognitive thing, something that only animals with brains can do. Since they will have dismissed sleep without a brain as not even possible, they would go for the other interpretation--a brain that does not sleep.
So even if we conclude that the submitted title really only has one "common sense" reading, that reading is the wrong one, so the headline is still a bad headline and should be changed.
Very interesting topic.