There currently is one option to travel by night train from Nice (France) to Paris (France), taking between approximately 13 and 14 hours: Namely the INTERCITES DE NUIT n° 5772 operated by SNCF.
In France, a few night train lines are operated by the state-owned SNCF.
In their night trains, they typically offer two types of carriages: seats and couchettes, with the latter available in first and second class.
Couchette carriages generally have 4-person compartments in first class and 6-person compartments in second class. The berths are simply designed in both classes and are about 1.9 meters long.
Yes, it's possible to reserve an entire couchette compartment as a private compartment. This can also be done for a group.
In first class, only four people share a compartment, the berths are wider, and travelers have access to station showers in Paris and Toulouse. Second class accommodates six people per compartment and allows bicycles to be brought for an additional fee of 10 euros – a plus for cyclists.
There is no dining car on board the Intercités de Nuit. It's recommended to bring your own food or eat in Paris before departure. It's possible to pre-order food and drinks. In first class, a small breakfast is offered. Each passenger receives a 'night kit' with a refreshing towelette, dental cleaning tablet, sleep mask, earplugs, and a small bottle of water.