BBC News has an article about a newly described condition called "aphantasia", where people can't visualize an imaginary scene.
If this is true, isn't it fascinating that we have apparently always had two groups of people: those (majority) who could "count sheep" in order to fall asleep, and assumed that everybody could, and those (minority) who thought that "counting sheep" was just some weird expression, surely not something actual people could actually do.
Personally, my mum once advised me to count sheep; I could visualize them jumping over the fence, but it didn't help much in getting me to sleep. Clearly the genes for this "aphantasia" are not linked to those for insomnia.