Though personally, I find the biggest difference in being able to sleep comes from avoiding screens and reading a book (or eink) instead.
I believe the issue is the frequency of light -computer screens tend to be on the bluer/whiter end of the spectrum -in fact if you look at apps like f-lux they try and help by changing your screen to be much more red; I'd also say that a monitor/tv is putting out substantially more light than your kindle. I'm also now wondering about the difference between looking directly at a lightsource (or reading from a backlit source) compared to indirect lighting; but once again I have neither the knowledge or references.
edit: (as I can't edit my original comment) I've just realised that I never mentioned suffering with extreme acid reflux so being hungry/empty stomach isn't really an option for me -and certainly stops me sleeping, which might well have a bearing on my anecdotal observations in the OP.