it's not getting enough light.

Mine do that more at this time of year, and if they don't get watered enough. Have some that probably get too much sun, but still drop their lower leaves. It's not serious enough to lose sleep over, tho' of course it keeps them from looking their best. If there isn't new growth starting at the base--which is less likely this time of year--I'd just cut back a few stems at a time. And I'd probably cut off most the bare stem and root the top--one reason I have too many plants to take the best care of them! When more growth starts at the base I'd cut some more. Works best, at least in my experience, to cut them all the way back rather than just part-way--and if you decide to cut back part-way be sure to pay attention to the direction the new buds face. The new branch will grow in that direction, and if you're not paying attention you'll get a lot of crossing branches going off in all directions.