[grumdan]

> What's the argument for night owls being better learners at night?

I would say that the argument is that many people report this to be the case and there are some studies pointing to the existence of such an effect (sometimes called chronotype). Even in the absence of rigorous studies, a reasonable default view is to say that such an effect may or may not exist and we just don't know for sure if it does.

I would argue that the burden of proof is on the person making the claim that this effect doesn't exist, as that's a stronger statement about the world. It also seems to contradict a lot of anecdotes without proper evidence that casts doubt on their accuracy; the simplified model that you mentioned, about sleep flushing toxins from your body may not capture all the relevant aspects here (for example genetic components to concentration ability).

Wikipedia references a few studies, but I'm not an expert and can't tell how conclusive or rigorous they are: https://en.wikipedia.org/wiki/Chronotype

[randomsearch]

I'm not asking for evidence, just a reasonable argument as to why people might learn better at night. Because the obvious thinking on that is, until very recently, night time meant most humans simply went to bed.

[grumdan]

I would say that variability in ability to concentrate might be reasonable to assume as a starting point, given how much people vary in other features as well, even if there are similar connections to how humans evolved. For example, food preferences are also in part a result of evolutionary processes, but vary a lot between people.