You are right about the day length. Adding a light to the coop (I use a timer to turn the lights on from 3 AM to 8 AM) helps, but I believe the colder weather is a factor too. If you were a hen, this wouldn't feel like a good time to start a family, right? Modern chickens are bred to lay a lot of eggs, but they still are affected by their more primal instincts about when to start and stop.
My niece was lamenting last night that she had orders for 8 dozen eggs this week, but only had 3 dozen to sell. That's averaging 5 eggs a day from over 100 laying age pullets. Many of these are young, but age-wise should be at POL now or soon. Since I'm not setting eggs (except Reese Legbars and turkeys) I don't really mind that production is down now, as long as it picks up in January.
So, for everyone that is disappointed in their flock's production, my advice is to view this as a time they rest and recuperate for a physically taxing season of laying that is ahead of them. At least that's what I'm telling myself.
The best "winter layers" in my flock right now are the Reese Legbars, I'm getting about a dozen eggs a week from 6 pullets. Not that impressive I guess, but I'm getting similar production from the 9 Rhodebars, and they were the ones that really impressed me last winter. In the Rhodebar's defense, they are a year older and some are still coming out of molt. It goes without saying that I'm very impressed by those Reese birds. If I could pick which group I would like to lay well now, it would be them.