Citrus are evergreen; they don't have a dormant period.
Meyers in particular are quite sensitive to significant light changes; it is will probably drop most, or maybe all its leaves to replace them with new leaves better suited to the new light. Continue to water it; and when it starts to make new leaves, give it a little food. When you put it back outside in the Spring, first move it to full shade; after 2 weeks to partial shade; and after 2 more weeks to full sun. In the Fall reverse the procedure and you will likely avoid the leaf drop.

is it getting enough light inside? south facing window or extra lights.

The 40's isn't low enough to damage a Meyer lemon. A lot of plants are stressed when you bring them indoors and will lose leaves. They can take light freezes while they are young and still in pots. Give it as much light as you can, and yes, water it.

In your area, if you are pulling them inside now, you don't have to graduate out of sunlight if you are sticking them directly in a south facing window or a very bright well lit area since the angle of our sun is past graduating into the shade intensity, believe me,well past. If you were bringing it in a month ago, that is a different story.

I would be much more concerned with the roots of your plant. Are you possibly over watering it? How long is it taking for your mix to dry out properly? Do you know exactly when to water?

By the way, once the root temps start dipping below 55 degrees, you will get a lot of yellowing although the tree will survive. Also, you will loose leaves if all factors are involved other than over watering such as, moving your tree from outside to a very dry atmosphere indoors, lack of fresh moving air, EXTREME light difference, and bugs

Mike

Thanks for the great info on my tree Leaves are turning yellow but are still on the tree. I will continue to water.

Thanks again,

Nancy