It is well known that FND patients do not sleep well, either due to digestive disturbances , pain or shaking. Whilst it is difficult to untangle what is the cause and what is the consequence in this process, the link below, gives some idea of how to improve sleep from a dietary point of view...favouring low carbohydrate meals...but it does not factor in any interference caused by FND.
There could well be more recent articles on this topic that do.