Starting with "At what time do you normally get in to bed?" to this Non-24 sufferer living in Germany is sort of like opening with "Enter your ten digit phone number" or "What is your ZIP code?"
How would you do it if you were us?
I no more have a time I go to bed every night than I have a US zip code or a ten-digit cell phone number.
We've taken what are the clinical standard measures - and some indeed feature counterintuitive questions. And you're right, one of the main issues with 'insomnia' is an unreliable sleep pattern.
We don't want to know how you slept last night, since this may be unrepresentative; but as you say an 'average' may also not capture it/be a natural way to report it. 3 x sample points ('last 3 nights') may be longwinded. Will consider...any ideas on a better question?