I have never been a consumer of any sort of caffeine (don't like hot drinks or fizzy drinks) and I have always struggled to get to bed or to sleep before, say,

I have never been a consumer of any sort of caffeine (don't like hot drinks or fizzy drinks) and I have always struggled to get to bed or to sleep before, say, 2am. I don't know whether it's genetic, though I'm not aware of any reason why it shouldn't be, but night-owl-ism is definitely not caused exclusively by caffeine. https://en.wikipedia.org/wiki/Delayed_sleep_phase_disorder#Mechanism https://en.wikipedia.org/wiki/Delayed_sleep_phase_disorder#M... seems to suggest that in extreme cases at least some of this is, in fact, genetic: it runs in families and there's a specific gene it's thought to be related to.