The mystery of why we need to sleep has perplexed scientists for decades, and evidence of sleep-like behavior has been seen in virtually every animal on the planet. A team at the Howard Hughes Medical Institute recently discovered signs of sleep in jellyfish, the first time such behavior has been identified in an animal without a central nervous system – suggesting that even organisms without brains may need some shuteye.

While sleep durations can vary significantly across different species, most research suggests that every animal that has evolved a brain, however complex, has also evolved the necessity of some kind of sleep state. Bullfrogs are often cited as one of the anomalous animals that don't sleep, but that conclusion, generated from a study in 1967, has recently been questioned.

There are many different theories as to why we need to sleep, but little research into how the function of sleep evolved. If we assume a brain is necessary for sleep then one could assume that primitive animals with no brain would display no signs of sleep-like behavior. This is where Paul Sternberg, co-author of the new jellyfish study, comes in.

"We wanted to figure this out once and for all," says Sternberg.

The team focused on the Cassiopea jellyfish, a small animal that spends a great deal of its time resting upside down on the sea floor. Like most jellyfish, the Cassiopea pulses its tentacles in a steady rhythm and the researchers set out to study how consistent this pulsing behavior was over a sustained period.

Recording the jellyfish for short periods of time at night and during the day, the team observed that the pulsing behavior did indeed reduce at night, but if food was dropped into the tank the activity increased. This behavior was the first sign that the sluggish night-time activity was due to a sleep-like state.

"It's like the odor of coffee permeating your consciousness in the morning," says Sternberg.

A system was then constructed that effectively disrupted the animal's floor-resting behavior by forcing them into open water at random times during the night. It was observed that at night it took the jellyfish three times longer to start pulsing after being disrupted than it did during the day. The researcher's note that this kind of groggy, delayed response is another indication of a sleep state in animals.

The final clue to whether jellyfish sleep came when the researchers completely disrupted this night-time rest state. For up to 12 hours they squirted jets of water at the animals, effectively keeping them awake all night. The next day the animals displayed noticeably reduced pulse activity, a sign of tiredness that returned to normal after a subsequent good night's rest.

Of course, it is fair to ask what we are actually defining as "sleep" when we reach the stage of studying animals without a brain or central nervous system. To meet the criteria for sleep, organisms must exhibit a period of reduced activity, in this state have a decreased response to stimuli that would otherwise arouse them, and when deprived of sleep show an increased drive to do so. The state the jellyfish seem to be entering into at night appears to tick these three boxes, and is the first time this kind of behavior has been studied in such a primitive organism.

The implications of the research are certainly fascinating, suggesting that the necessary biological functions of sleep evolved earlier than the brain or central nervous system. And if sleep is a state that operates on a deeper level than just in the brain, then what other organisms could exhibit sleep-like activity.

"And perhaps a more far-fetched question," asks Ravi Nath, one of the study's co-first authors, "Do plants sleep?"

The study was published in the journal Current Biology and the team discuss their research in the video below.

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