I always use a top sheet. Wouldn't wash a duvet weekly.

We've experimented a bit since we sell a number of natural bedding and linen lines. In winter we definitely prefer a top sheet (the more layers, the better) but as the weather becomes warmer, tend to "go European" and just use a duvet.

A good organic cotton duvet cover usually works fine but if you find layers too hot, you may want to consider an organic linen duvet cover. It's more absorbent and breathable so will sleep cooler.

Not inexpensive for sure but we heard back from a recent customer who got a set of organic linen chambray sheets and said they were more comfortable temperature-wise.

Link is to an organic linen chambray duvet cover in the same line: https://www.coyuchi.com/organic-linen-chambray-duvet-cover.html