Human may have more in common with jellyfish than previously thought.

Scientists from Caltech have found that primitive jellyfish Cassiopea actually starts and ends its day with sleep, implying that sleep may be an ancient behavior, largely untouched by millennia of evolution.

“It may not seem surprising that jellyfish sleep—after all, mammals sleep and other invertebrates such as worms and fruit flies sleep,” Ravi Nath, the paper's co-first author and a graduate student in the Sternberg laboratory, said in a statement. “But jellyfish are the most evolutionarily ancient animals known to sleep.

“This finding opens up many more questions: Is sleep the property of neurons? And perhaps a more far-fetched question: Do plants sleep?”

In order to sleep an organism must demonstrate a period of reduced activity or quiescence, must exhibit a decreased response to otherwise-arousing stimuli while in the quiescent state and must show an increased sleep drive when it is deprived of sleep.

“When humans sleep, we are inactive, we often can sleep through noises or other disturbances which we might otherwise react to if we were awake and we're likely to fall asleep during the day if we don't get enough sleep,” Claire Bedbrook, co-first author and a graduate student in the Gradinaru laboratory, said in a statement. “We might seem extremely different from jellyfish but we both exhibit a similar sleep state.”

The researchers set up a system of cameras to monitor the jellyfish and found that they go through periods of inactivity at night—pulsing about 39 times per minute at night, while pulsing about 58 times per minute during the day.

They then set a jellyfish on a platform higher up in the tank and pulled the platform out from underneath once it showed signs of quiescence. Normally, an alert jellyfish would immediately swim to the bottom of the tank but the jellyfish in the sleep state floated in the water for up to five seconds before it “woke up” and reoriented itself.

Next the researchers set out to prove that when deprived of sleep the jellyfish would exhibit an increased sleep drive—similar to when humans suffer through a sleepless night. To do this they pulsed water at the animals every 10 seconds for 20 minutes to keep them awake.

The jellyfish were more likely to fall into quiescent state during the day than they normally did after a night like that.

While the sleep behavior exhibited is similar to humans, the genetic mechanisms that underlie sleep remain unknown.

“Many animals have the same genes that govern sleep,” Michael Abrams, co-first author and a graduate student in the Goentoro laboratory, said in a statement. “Though it was beyond the scope of our project to measure gene expression in jellyfish, we tested the effects of compounds that in other animals are known to promote sleep, such as melatonin.

“We found that these compounds did affect jellyfish sleep in the predicted ways, suggesting that their underlying sleep mechanism is similar to those of other organisms—including humans.”