I've been thinking about this because I'm requesting a change of work pattern, and citing chronic insomnia as one of the reasons for it. I imagine the HR department will want to categorise it in some way.
I think of it as a physical disability that impacts my mental health. It is circular, because stress and anxiety do trigger the insomnia, but they're not the only triggers. Body clock, daylight, birdsong and hayfever are just as culpable.
Any thoughts?