Citrus roots need to be above 55F to function. Don't forget, soil is usually colder than the room its in, so just because the room is 60F doesn't mean the soil is!

When light hits the leaves, it heats them up. The basic defense for the plant to ensure the leaves don't get too hot and damage the plant is to send water to them - this cools the leaves. If the roots are too cool and not functioning, they can't send water to the leaves. The leaves therefore cannot cool down. To respond, the plant drops the leaves to prevent damage and strain.

So, if light is hitting the leaves but the roots aren't working (due to being too cool) leaf-drop occurs. If the citrus are to be in a cool room, they will be fine with little or no light. You could put them in an unlit garage over winter, as long as the roots stayed cold. They should be perfectly fine under these conditions. They won't grow, but they will live ok through the winter (just don't let it get to freezing point)

I keep my citrus in a south facing window over winter. As a result, the room needs to be quite warm so the soil stays warm so the roots will function. I have experienced absolutely no leaf drop at all, and several of my citrus are going through flushes of growth.

So if they are in a cold, unheated room (under 60F) make sure they are in minimal/no light. If they are in a warm room, they need the sun.

Thank you aesir22! That answers the question I had.

Does this translate into any rule of thumb concerning when to put an indoor lemon outside in the spring?

Wait until dangers of frost have passed. Over a 3-4 week period, slowly introduce it to the outdoors. So, put it in a shaded location for a few hours each day, then bring back in, gradually increasing the amount of hours per day it goes out. Then gradually increase the light factor, so go slowly from shaded position to the location you want to keep it.

If the temps are going to stay below 55F (65f to be safe) it should either be kept in until weather warms a little, or put in a position where it will not get direct light (either a shaded location or put a shade cloth over it to dim down some of the rays.

Yes..T
Thanks alot aesir22!!. It makes alot of sense. In fact I have followed your advice. What a difference it made on my plants. They seem to love warmer roots and are holding the leaves and still taking up nutrition.
Again, thanks.