There currently is one option to travel by night train from Tarbes (France) to Paris (France), taking between approximately 12 and 13 hours: Namely the INTERCITES DE NUIT n° 3740 operated by SNCF.
The National Company of the French Railways (SNCF) operates a handful of lines throughout continental France.
In their night trains, they typically offer two types of carriages: seats and couchettes, with the latter available in first and second class.
In the couchette carriages, you travel in 4-person compartments (first class) or 6-person compartments (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.
You should bring your own food as there is no restaurant on board. Alternatively, you can pre-order meals and drinks or eat before departure. First class travelers receive a small breakfast. Each passenger receives a night kit with refreshing towelette, dental cleaning tablet, sleep mask, earplugs, and water.