Depending on the type, I believe it is going dormant and that is why its's dropping leaves - taking in account that your care and soil type is fine.

Mine always lose it's leaves when I bring it in right before frost. All these years, I thought it was because of the lower light but have read on here that it's natural for them to lose leaves and go dormant.

Thank goodness I didn't throw it away. I just figured I could wait until spring for it to have leaves since the trunk was always healthy. And that is what I kept it.

Mine will often shed leaves with the transition of moving to the inside. Can be a sign of change of conditions or dormancy.
Shedding of leaves can just be a natural process of sinensis, also.
Usually shedding from the lower extremities first and possibly moving upwards to tip.
I keep mine in south facing light while inside and most do not lose their leaves.
A good sign to reduce watering though when this happens.
The little guys will benefit from stronger light levels than the older plant atleast for their first winter.
Rick

Thanks guys! Seemed a little sudden for dormancy. It still gets about 6+ hrs of sunlight per day, but the temperature has definitely changed. I'll be bringing it inside with the rest of my seedlings soon since temps are supposed to reach the high 40's by the end of the week.