If you're safe.

You definitely see a bit of this in other primates. They sleep on trees (much safer) and for a lot more hours than humans.

That's great for primates, but what about nematodes? Everything with a nervous system sleeps, even when the cost is high.

Obviously sleep is necessary. I'm saying that there isn't always a large pressure to minimize sleep. Long sleep can be beneficial just to preserve energy.

Right, but my argument is "high pressure never managed to push past this barrier, therefore it's a strong barrier." The existence of other, lower pressure circumstances doesn't have any bearing on the conclusion -- unless you want to argue that the low pressure circumstance is so universal that the high pressure circumstance doesn't exist, that no animal anywhere is inconvenienced by the need to sleep. But I don't think that is what you were going for.

Sleep has to be essential to life as we know it, because no animal has completely outevolved it. Some animals sleep an hemisphere at a time (dolphins, long-flight birds) or in fragments (giraffes), but none truly goes without sleeping no matter how challenging their environment is for a nap.