Unless it's on Film 4 or some other commercial channel, which gives us handy cop-out sessions in the form of commercial breaks. It's one thing to leave a film when it's in full swing, quite another to leave when the programmer has decided to break it into sections and give us ads for insurance companies in the breaks.
Novels have breaks too, in the form of chapters. As writers, we want to keep our readers glued to the book late into the night, so we don't want them to use the excuse of the chapter break to put the book down. This means we have to think about the ending of each chapter very carefully.
Does it make the reader want to read 'just the first page' of the next chapter? Once we've got them reading the first page of the next chapter they're bound to carry onto the next, and the next. Ways to make the reader stay with the book are by using what could be crudely called cliffhangers. Examples of a cliffhanger could be the arrival of an unexpected character or the making of an unexpected announcement. It could be something that is unresolved, a niggling question, or a statement that is left hanging in the air.
It doesn't matter what it is, so long as it doesn't help the reader put the book down. Examples of things that help you put the book down would include questions resolved without new ones being raised or characters going to sleep (this works by suggestion).
But chapter ends are only the most visible form of break. There are usually spaces after scenes, so you really need some hook to get people reading on after a scene ends. And readers can break away from the text mid-scene for example to check something someone said earlier, or if they're puzzled over the use of a particular word. Any time a reader gets confused and pulls away from the story world you've created for your novel then potentially you've lost them.
Your writing should be so smooth that before they know it, they're the ones stuck on the sofa, bug-eyed and desperate for bed, but unable to stop reading. In effect, your aim as a writer is to induce sleep deprivation in your readership.