Do all animals sleep? The strange case of the jellyfish

Research Summary
Efrat Magidov and Yuval Nir, PhD

Do all animals sleep? This question has been hotly debated in the sleep research field (for example, in these two perspectives). Apart from being an interesting question for biologists and curious folk, it is an important avenue of research for anyone trying to understand why we sleep and develop treatments for sleep disorders, even in babies. That’s because in some simple animals such as drosophila (fruit fly) and C. elegans (worms) scientists can rapidly screen for genetic mutations and test possible drug interventions, so if indeed these animals sleep we could potentially gain new important insights about sleep and develop new treatments for sleep problems.

But, how can we tell if a certain animal sleeps? When we think of our own sleep, it seems natural to assume that all animals that sleep will have specific periods in the night or during the day when they are with closed eyes, maintain a certain immobile bodily posture, and that their brain activity will show large slow waves as occurs during most of the night in human EEG. But sleep comes in different forms across the animal kingdom. Some animals can sleep with open eyes and standing up (horses) or while moving (dolphins), others may sometimes sleep in fragmented intervals equally spread across the day and night (ground squirrels). Sleep can be associated with small and fast brain waves (fruit flies), and some animals can go on for several weeks without any sign of sleep (birds migrating or mother whales after giving birth). Is it possible to sleep without eyelids, a place to lay down on or even without a brain? Does a plant opening its flowers during the day and closing them at night “sleep” or not really? 

To answer if a certain animal sleeps, we must first clarify what qualifies as sleep. Like many scientific questions, the answer depends on the way we define sleep. For example, if scientists wish to determine if a certain organism is conscious, they first need to define what consciousness is, and then suggest criterions by which they will test if the organism fits into this definition. Fortunately for sleep scientists, defining sleep is much easier than defining consciousness, since sleep in its nature is a behavioral state, and can be identified and measured objectively. Most sleep researchers agree on three main characteristics that make up the conditions that need to be met for deciding if an animal sleeps: (1) reduced mobility, which is reversible (you can “wake up” from this state, unlike a state of coma); (2) decreased sensitivity to sensory stimulation during this period (a state of “disconnection” from the outside world), and (3) signs of compensation if you are deprived of this state (i.e. if you don’t sleep when you usually do, you’ll sleep more later on). This is called a homeostatic process and indicates that the organism is carefully tracking how much sleep it gets to ensure there is sufficient sleep (in contrast to just lying down and resting). Even with these definitions in place, there’s ongoing controversy in the sleep research community if all animals sleep, with some exotic animals like the bullfrog that may tip the scale in favor of one view or the other.

The strange case of the jellyfish. In the last 20 years, sleep scientists found that even relatively simple animals sleep including flies, worms, and sea slugs, even if it may not seem so upon first glance. This year, a group of researchers from the California Institute of Technology (Caltech) took it one step further by testing if the jellyfish Cassiopea sleeps, and reported their findings in an article published in Current Biology. What makes jellyfish an interesting test case is that it has no brain or central nervous system; rather, jellyfish have a group of interconnected neurons (nerve cells). This small creature lives upside down on the sea floor, and its body pulsates at a steady rate of about one time per second. First, the researchers observed that the frequency of body pulses decrease significantly during the night, and that food delivery to their tanks reversed this state and made the jellyfish “wake up”, thereby meeting the 1st criterion for sleep.  The researchers went on to check the existence of the other two sleep characteristics. To test their sensory responsiveness during nighttime rest periods, the investigators lifted the jellyfish from the bottom of their tanks and measured the number of body pulses it took them to reach back to the tank bottom (their ‘responsiveness’). During sleep-like states, it took the jellyfish three times longer to return to the tank bottom after such “lift-up” stimulation, demonstrating that they are less responsive to external stimulation. So, jellyfish immobility is not merely like a “couch-potato” resting state when we can respond quickly to events in the environment if needed; it’s really like sleep, when we are disconnected from the world around us. Finally, the researchers showed that these small creatures really need the rest periods; if the jellyfish were sleep-deprived during the night by delivery of brief pulses of water into their tanks, they had more sleep-like behavior during the following day, just like a shift-worker at the end of a long red-eyed night, finally catching some sleep during the day.

Possible implications. By studying sleep in this exotic creature, the research team concluded that sleep appeared earlier in evolution than had been thought before, suggesting that basic cellular and molecular properties (such as producing Melatonin) are more crucial to sleep than, for example, having a developed brain. It suggests that individual cells, rather than whole brains, may be the ones that “need sleep”, perhaps to be able build large molecules – a feat that is difficult to achieve during intense activity. We can only imagine what sleep ‘feels like’ for aquatic wonders with tentacles and no brains. At any rate, the very idea that these animals sleep highlights that sleep is a universal and evolutionary-preserved behavior that is essential for our health and well-being.

Further reading.

Jellyfish caught snoozing give clues to origin of sleep

Caught napping: Snoozing jellyfish prove a brain isn't necessary for sleep


Dr. Yuval Nir and Efrat Magidov conduct research on sleep at Tel Aviv University. More information on their work.