We start ours before the fall equinox. Lights are on a timer. My horse is brought in around 4:30, so that's when her light comes on and it goes off at 10:30. She's also blanketed when it gets cold because she doesn't have a winter coat.
60s = sheet
50s = blanket
40s & below = sheet and blanket
Blanketing alone doesn't have an effect on the amount of coat a horse grows. Some horses don't grow much of a winter coat and some are puffballs.