I think my animals know Winter time. They seem to all be rather lazy doesn't help its 20-30 degrees out last 2 weeks. I live up in MA so maybe people around my area wait longer? They can see the sun enter and leave room as well so they know days are short.
The days are still short but sunset has been getting later since the first week in Dec. (even though the solstice is 12/21, actually between 12/7 or so and 12/21 the day's duration is getting shorter but sunrise and sunset both get later). I put my geckos together just after New Year, though I don't expect there to be much action for a month. The males are definitely getting interested and I think I just had my first mating last night with one who ovulated early (first-timer). I have another, though that I know probably won't ovulate till April.