When you get your first waist trainer, you will want to wear it all the time. It feels great, you look great and you can feel it working. There is a large portion of the time you have not been wearing it though, at night. So can I sleep in a waist trainer?
From anecdotal evidence, it seems to come down to the individual. Some people can sleep wearing a trainer comfortably, others cannot and find it too restrictive. If you plan on sleeping in your trainer, it might be best to get an adjustable one, or a looser one to start with. You can then progress to a tighter model for day wear.
Take things slow and work your way up, rather than jumping in at the deep end. Losing out on sleep will have a detrimental effect on your health, and will wipe out all of the benefits you get from the device. As always, listen to your body and seek medical attention before making any serious changes to your regime.