Definitions
Insomniacs are people who feel tired and need sleep but just can’t seem to fall asleep. It’s a disorder and leaves the insomniac feeling even more exhausted and destroys their quality-of-life and negatively affects their health.
Nonsomniacs are people who get by just fine with little to no sleep. They don’t feel tired and live on very little sleep (typically <1-2 hours per day) without any adverse effects.
Past research
In 2016, researches did some experiments, using fMRIs to examine the brains of people as they sleep. They found that when we (and animals) sleep, the neurons physical shrink. This creates more space inside the brain, which allows CSF to flush away the metabolic-waste created throughout the day by neuronal activity.
Results and inferences
This seems to be the ultimate purpose of sleep (though it doesn’t explain dreams, which is a different topic).
Future experiments
It can be confirmed as the purpose of sleep by analyzing the neuronal activity, metabolic-waste, neuronal shrinkage, and sleep patterns and quantities of different animals. Hopefully, someone will actually do that research since it would fairly definitively prove that the evolutionary purpose of sleep is indeed to clear the brain of waste.
On a related note, it would be handy to examine the brains of nonsomniacs. There are two likely explanations of why they sleep so little. It could be that they have naturally narrower neurons, so their CSF can flush most or all of their neuronal metabolic-waste all day without needing to induce sleep, or it might be that their neurons just produce less or no metabolic-waste than most humans, so there’s less to flush.
Performing the same 2016 study on nonsomniacs could/would explain both nonsomnia and strengthen the original study’s findings.
Optional studies
It’s unlikely since it’s probably a different issue, but it might also help with insomnia research.