Sleep onset is a creative sweet spot (science.org)

That sleep stage is called 'hypnagogia' and there is an interesting book from 1987 by Andreas Mavromatis where the author investigated it from multiple perspectives. There is a copy at https://archive.org/details/hypnagogiaunique0000mavr for everybody's reading pleasure. See also https://en.wikipedia.org/wiki/Hypnagogia for reference.