Snoozing jellyfish have confirmed that a brain is not necessary for sleep.
Scientists made the discovery after observing a primitive jellyfish called Cassiopea that lives upside down on the sea floor and lacks any kind of central nervous system.
The creatures, which resemble miniature cauliflower heads, have bodies that pulse with a steady rhythm.
Videos of the jellyfish taken over 24 hours showed that they pulsed less frequently at night.
But they could be “woken up” by dropping a little food into the tanks where they were kept.
“It’s the first example of sleep in animals without a brain,” said US researcher Professor Paul Sternberg from the California Institute of Technology (Caltech).
He compared the effect of the food to that of smelling coffee in the morning.
A second sign of sleep was seen when the jellyfish were physically disturbed during their low-activity periods, forcing them to float in open water.
At night it took them three times longer to recover and return to a resting position. Such “grogginess” was typical of sleeping animals, said the scientists.
The researchers also found that the jellyfish slowed down during the day if they were kept “awake” by having tiny jets of water fired at them at night.
Graduate student Claire Bedbrook, another member of the Caltech team, said: “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.
“We might seem extremely different from jellyfish, but we both exhibit a similar sleep state.”