In the following situation, does "very" or "so" sound more natural?
You usually go to bed early. Last night you went to bed very late (for you) and as a result you are very tired this morning.
Friend: You look tired this morning.
You: Yes, I'm not used to going to bed ____ late.
I answered "very", but the answer in the book (English Grammar in Use, Unit 61.2.4) is "so".
What is the difference between "so" and "very" in this context?