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I hope it is not too late. I just saw your message.
It appears to me to be a watering issue. I really do not think light would cause the wilting that I see.
First I would check for insects on the trunk, branches and leaves - even underneath the leaves. The pests most likely to be on citrus are aphids and scale.
Next, if you can, put it in the shower and spray it off well which will help control insects and soak through the soil well. If you did see any pests I would first spray thoroughly with insecticidal soap (aphids) or horticultural oil (scale or aphids). Be sure you get the underneath the leaves. Then follow up with water - just barely warm water. After the leaves dry off then put it back where it was. Do not water again until the soil seems slightly dry when you stick your finger in up to the first knuckle. When watering, water enough so the soil soaks in and a little comes out the bottom of the pot. I have never misted my citrus and do not see any reason to do so.

I have two lemon trees, one navel orange, one key lime, and two grapefruit. One lemon tree I have had for years and moved with me to Alaska and back.

Here is a link that might be useful: Edible Landscaping - lemons