The overnight train journey between Paris and Madrid is made onboard the 'Francisco de Goya" sleeper train. This Elipsos Train Hotel is effectively a hotel on rails, with a full restaurant and bar. There are a range of seating and sleeping options, which you can tailor to your budget. The Elipsos website offers a virtual tour of the train so that you can get a better idea of what to expect.
In the middle of the night, the train pauses at Hendaye to switch gauge. This process doesn't involve passengers getting off the train (unlike some daytime alternative routes), but the wheels are changed with passengers on-board.
As with any night train, it can be difficult to get to sleep if you are a light sleeper. If you are worried that you might get woken up, take a pair of earplugs, which should help. We recommend paying for a couchette or sleeping berth, rather than trying to sleep in a reclining chair. Berths are significantly more civilised, offering greater security, comfort and overall level of service. Surprisingly, they can also be cheaper than reclining chairs, as the operator offers special reduced rates on berths, but not for reclining seats (view the pricing section for more information).