Quote 1
"Human beings have to have their sleep." (1.5)
Is Kafka suggesting here that Gregor turned into a bug because he didn't get enough sleep? That's a good one to try on your parents or anyone else the next time they try to wake you up early. In any case, you'll find Gregor making all kinds of statements in the story that are ironic because of the context. That is, what would normally be a obvious statement – everyone agrees that human beings need sleep – becomes absurd in view of the fact that Gregor is a bug. (This, by the way, is what literary critics call "dramatic irony.")
Quote 2
Gregor's legs began whirring now that he was going to eat […] "Have I become less sensitive?" he thought, already sucking greedily at the cheese. (2.7)
Gregor's new body responds in strange new ways to his needs. (Imagine how awkward it would be if your legs spun around every time you got hungry.) Before you dismiss Gregor's habits as another indication of his disgusting vermin-hood, take a look at the way the boarders eat in 3.10-11.
Quote 3
When Gregor Samsa woke up one morning from unsettling dreams [...] "What's happened to me?" he thought. It was no dream (1.1-2)
The philosopher René Descartes once wondered how it was possible to distinguish between dream and reality. His answer? God wouldn't be so cruel (read more). Kafka plays on the Cartesian reference here. How does Gregor know it's not just a dream? Isn't being a bug sure evidence that he's stuck in a dream? Would God be so cruel?