Many, but not all, long haul aircraft have 'secret' (ie: not readily visible to passengers) areas for crew to take breaks during long flights. Some are in the space above the cabin. In the 747 they are sometimes behind the upper deck cabin. Some are below the main cabin floor and some in an isolated area on the main deck floor. There are usually separate rest areas for pilots and for cabin crew.
Those areas are called "crew rest areas". If you search for that term, you'll find lots and lots of web pages, often with pictures, of various accommodations on various aircraft.
Note that the provision and nature of crew rest areas are usually optional extras that each individual airline will choose (or not) when they purchase a new aircraft. The Boeing 787, for example, has provision for three or four or more sleeping bunks above the main cabin, and most airlines have ordered that, but the Australian Jetstar 787 aircraft don't have crew rest areas.