Can Invisalign be used only at night time?

Invisalign and braces in general work best when a constant force is applied to the teeth causing tooth movement.  If you are not compliant in wearing your Invisalign trays constantly then I would not recommend them.  Your teeth will be in a state of shifting during the evening and shifting back during the day when you are not wearing them.  Your treatment will either progress very slowly or not at all.  

Invisalign works by having constant force on your teeth for 20 hours a day.  You essentially act like they're glued to your teeth like braces would be.  If you sleep in them only, the teeth will move at night but will move back to where they were during the day resulting in net-zero.