Snoozing jellyfish deepen the mystery of why we sleep

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.

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