As a follow up question - were artificial sources of light - candles, lamps, etc - prohibitively expensive for the majority of the population? Would - say - a medieval farm labourer go to sleep when it got dark?

You may find this thread to be of interest: in it, u/zagreus9 comments on one theory by A.R. Ekirch about sleep patterns in pre-industrial times, namely that people used to experience "segmented sleep," i.e. sleeping in shorter intervals and waking up in between.

However, be aware that u/whatthefat offers a contrary view here, in an excellent post dealing with the evidence for Ekirch's theories.

The thread I link to is rather old, so it may well be that more recent research exists.

Always more to be said on this, but definitely check out /u/hillsonghoods response to a similar question here.