How do horses "know" when to grow their winter coat?

Horse people joke that horses can tell when it's going to be a bad winter and grow more fur. That's not true, although it sometimes seems to be.

The actual trigger is light levels - which is why show horses kept under lights don't grow as thick a winter coat (this is often desirable in horses that are working hard through the winter, as otherwise you have to start shaving them so they don't overheat). Horses grow their winter coat when the days get shorter and shed it when they get longer again.

Actual winter coat length is dependent on light levels (which happen to nicely equate to temperature) and genetics - some horses will grow almost no winter coat and others become very shaggy when kept in the same conditions. Thoroughbreds and Arabians (and some Quarter Horses with a lot of TB or Arabian blood) tend to grow very thin coats, whilst ponies, like this Shetland, get very fuzzy indeed.

Image source: Miles Wolstenholme via Wikimedia Commons.