If you are from the UK you need to buy an Interrail pass, not a Eurail pass.

Night trains are included, but you need to pay a reservation fee for a seat/couchette/sleeper in order to travel with the train.

The ticket covers the travel-price. The additional fees for bed/sleeper/couchette/seat REServ etc have to be paid as also a normal travller would need to do

Plus that the nr of such sleeper trains is quite limited-better do reserach first if you plan extensive travel