Check the cold hardiness of the plants you are overwintering but 40 degrees is probably fine for most plants.
Sumire's greenhouse is full of citrus and a few hardy orchids. She keeps hers at 40 but it often dips to close to freezing in really cold weather. Her plants always do great but are all hardy to zone 8.
I grow tropical orchids so try to keep temperatures between 55 and 60.
