In rural Brittany, people simply didn't do much - stayed indoors, saved fuel by staying in bed, led hard lives so enjoyed an extended kip. They didn't have altered biorythms like hibernating animals but if you think about it, the stuffiness inside and reluctance to go outside would make them a bit more sleepy than usual, plus the longer nights / shorter days. it was just a pragmatic response to there being no jobs you could do outside, very little daylight to do anything requiring a bit of light like weaving, and a nned to live frugally.
I haven't read the links yet but I'll do that now.