Scientists say our ancient fish-like ancestors developed “slow-bursting sleep” before crawling out of the ocean.
Although today’s fish are unable to close their eyes (because they lack eyelids), changes in their brains and muscles are the same as those that take place in humans.
Scientists already knew this occurred in mammals, birds and reptiles, but the fact it has been recorded in a zebrafish suggests it is a very ancient adaptation.
“While modern mammals have their first ancestor 70 million years ago, modern fishes derive from ancestors living 450-500 million years ago,” senior author Philippe Mourrain, from Stanford University School of Medicine, told The Independent.
“The fact that we find sleep brain and body dynamics similar in both zebrafish and humans strongly suggests that these neural and muscle signatures were already present in our common ancestor more than 450 million years ago.”
Like humans, fish lose muscle tone, their heartbeat drops and they do not respond to stimuli when sleeping. The only real difference is a lack of rapid eye movement during REM sleep, researchers found.
To study the fish, they built a fluorescent light-sheet microscope that could image the whole body in extremely high resolution.
They recorded brain activity as the fish slept and observed heart rate, eye movement and muscle tone, according to the study, published in Nature.
Although scientists cannot know for certain that all animals sleep, it appears to be a universal need.
The exact benefits are still a mystery but people who fail to get enough sleep suffer memory lapses and impaired judgement as well as a higher risk of obesity and high blood pressure.
Sleep disorders are also linked with neurological diseases such as Alzheimer’s and Parkinson’s.
“This research is critical as it strengthens the idea sleep has core functions that are conserved throughout animals such as maintenance and remodelling of neuronal connections,” said Dr Mourrain.
Scientists say understanding sleep functions in animals could help us better understand the consequences of sleep disruption.
“Because fishes’ neural signatures are in essence the same as ours, we can use information about them to generate new leads for drug trials,” said Dr Louis Leung, from Stanford University School of Medicine.
Mice – often a stand-in for human research – are nocturnal and a less relevant model for our sleep.
“As zebrafish are diurnal like humans, it’s perhaps more biologically accurate to compare fish sleep with humans’ for some aspects,” Dr Leung said.
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