It's not a question you might have thought about, but which came first: sleep or the brain? To put it another way, can organisms without brains go to sleep? Based on a study of primitive, water-dwelling life forms called Hydra vulgaris, scientists might finally have the answer.
New research shows how these tiny creatures can enter a sleep-like state despite not having brains – and that could teach us a lot about how animals evolved the need and capacity for sleep.
What's more, the team behind the study found that the chemicals that bring on drowsiness in human beings had the same sort of effects on Hydra vulgaris, suggesting a biological link across the species, despite our vast differences.
"We now have strong evidence that animals must have acquired the need to sleep before acquiring a brain," says biologist Taichi Itoh, from Kyushu University in Japan. "Based on our findings and previous reports regarding jellyfish, we can say that sleep evolution is independent of brain evolution."
Hydra vulgaris. (Przemysław Malkowski/CC BY-SA 3.0)
The research mentioned on jellyfish is one of several previous studies that has found sleep-like behaviour in organisms without brains. However jellyfish, which have a more advanced arrangement of nerves than Hydra vulgaris, do seem to follow a circadian rhythm.
The team weren't sure what they'd find in the simpler Hydra vulgaris specimens, which have nerves, but only in a primitive, decentralised organisation
Using a video system to track movement – and a light flash system as an alarm clock – researchers observed the little creatures going into sleep cycles around every four hours on average. It seems they get worn out a bit quicker than we do.
Exposing the hydras to melatonin and the inhibitory neurotransmitter GABA – both of which bring on sleep in many animals, including humans – got them 'snoozing' for longer. In contrast, exposure to dopamine, which usually wakes us up, actually increased sleep activity in the hydras.
These reactions give scientists more clues as to how sleep may have evolved in the animal kingdom, both before and after living organisms developed a brain. It's not clear whether the disorganised network of nerves inside Hydra vulgaris is playing a role or not.
"While some sleep mechanisms appear to have been conserved, others may have switched function during evolution of the brain," says Itoh.
Further experiments with vibrations and temperature changes disturbed the sleep of the hydras as you might expect – much as we might, after being disturbed they slept longer the next day, and sleep disruption even interfered with cell proliferation.
The expression of 212 genes were altered by this sleep disruption, including one called PRKG: a protein linked to sleep patterns in many animals, including mice and fruit flies. There's a possibility that further study could reveal genes that we don't yet know are connected to the sleep process, in a whole range of animals.
Our own brains have gone through more changes than you might think, and scientists are always making new discoveries about how the brain evolved – and the role it plays in telling our bodies when it's time to sleep.
"Many questions still remain regarding how sleep emerged in animals, but hydras provide an easy-to-handle creature for further investigating the detailed mechanisms producing sleep in brainless animals to help possibly one day answer these questions," says Itoh.
The research has been published in Science Advances.