Evolutionarily, sleep is very expensive. It takes us out of commission and makes us vulnerable a third of the time or so every day.
The amount of sleep that someone needs varies. Some people need nine hours a night to function. Other people need six. So there is a range of sleep need in the general population, and is needing more sleep was en evolutionary disadvantage, the selective fitness advantage of having more sleep would cause people who needed less sleep within the range of variation to increase their share of the gene pool, and the people who need more sleep within the range of variation to decrease their share of the gene pool. But the amount of sleep people need has stayed fairly stable for tens of thousands of years or more.
And, there are ways in nature that we observe that people wouldn't need so much sleep. Lots of grazing animals, like antelope, have one half of their brain asleep and one half of their brain awake at one time, much of the time. So, even though each half of the brain gets sleep similar to humans, the time that both halves of their brains are asleep is much smaller. So we know that being completely asleep in both halves of your brain at the same time isn't an anatomical necessity (although maybe having both halves of your brain working at once when you are awake is more important to humans because of the way that they use their brains than it is for grazing animals).
What selective fitness advantage are people getting from needing more sleep?
Here's a personal conjecture. A hypothesis.
If you are completely vulnerable for a third of each day, you can't afford to be a loner. You need to live in a community where you trust other people to look after your safety when you are asleep. You need a community with at least several adults who can watch over the community when you are asleep and you need enough adults awake at any one time to be a credible deterrent to whatever threats your community faces.
People who sleep a third of the day are forced to live in communities like this, a minimum viable group size. People who sleep little or at all aren't forced to live in communities like this and may prefer to be loners and to not have to deal with the demands of social life that has costs of its own.
But there is a large fitness premium for living in decent sized communities relative to being a loner, far beyond the benefits is just having someone to look out for you when you are vulnerable while you are asleep.
So maybe one of the big evolutionary benefits of needing a lot of sleep is that it forces most people to live in at least modest sized communities, which increases their selective fitness in all manner of ways.