Thanks, shivvy... your observation is the reason I continue to visit the site. I suspect that many of the plants you see in shops are under lighting systems that provide artificial sunlight. If the plant was on the equator I don't think it would go through the photoperiodism process. I'd qualify that by saying that guess would apply to plants that were bred and raised in that environment. I'm not an expert in botany, but I've experienced the same reaction with some indoor plants as you are with your lemon...
By the way, if my advice is correct, in the spring when the plant comes back, check to see if there are any whitish deposits on top of the soil. If there are, a neighbor that raises lemon plants advises that the potting soil needs flushing. She does this about 3 or 4 times a year. She simply takes the plant, contianer and all outside and lets a large volume of slow running water flood the plant. she uses a rule of thumb of 4 times the volume of the container in which the plant resides. The "salts" are from fertilizing and some residual minerals contained in almost all water. She has experienced the leaf drop we are discussing as well. She does keep the plant in a cool, semi-darkened room to simulate winter conditions and keeps the plant away from dry, warm heat such as comes out of our central heating systems... She also advises that misting the plant when it starts up again in the spring...Good luck!