Why don't we wake up when a spider bites us in our sleep?

Question

Why don't we wake up when a spider bites us in our sleep?

Answer 1

We don't feel it, as when a person is sleeping, they can be in a deep sleep and never feel or hear anything.

Answer 2

Some people do, actually. It really depends on the person. Usually, spiders bite late at night, when we are in REM phase...in other words, deep sleep. We don't feel the pain. Some of us don't realize we've been bitten until late the next morning.

Also, some spiders have anesthetic in their venom, that makes you "not feel" their bite.