There currently is one option to travel by night train from Paris (France) to Tarbes (France), taking between approximately 10 and 12 hours: Namely the INTERCITES DE NUIT n° 3731 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.
First class berths are slightly wider than in second class and there are only four berths per compartment. Advantage of first class: In Paris and Toulouse, there are showers at the stations for first class passengers. In second class, six travelers share each compartment. For cyclists, it's possible to bring a bicycle in second class for an additional fee of 10 euros.
The night trains don't have a dining car, so it's advisable to bring your own food or eat before departure. You can also pre-order salads, sandwiches, and drinks. First class passengers receive a small breakfast. All travelers get a night kit with refreshing towelette, dental hygiene product, sleep mask, earplugs, and a bottle of mineral water.