Native AmE speaker here.
I know a long time has passed since this was posted, but...
For me, the "up" in "up until" can be redundant, can be more informal, but is used in certain circumstances more than others because it does have a purpose. Just "until" is stating the time that something ended, but doesn't necessarily lead the listener into what happens next.
The focus of "up until" has to do, not with emphasis on the time, but with focusing the listener on the following event or the conclusion of the current action, as well as the time that the current action being spoken occurred. The following action isn't directly stated, but implied, at times. It also has a tendency to be used in surprise at the conclusion of an action.
If I use "up until" naturally, it really comes out if I'm talking about my past, like:
"Up until I was 15, I went to Idle High School, and then I transferred to West Lake High."
Or in something surprising happening recently, especially something you've learned:
"Up until Todd told me, I had no idea that your birthday was the same as mine!"
In the first one, the following event is directly given: "I transferred to West Lake High." In the second one, the following event is implied: "Now I know!"
You can also look at this in an example of someone knowing the following event and using "up until" to emphasize the conclusion of that action:
A: "Hey, I heard Louise decided to go back to university."
B: "No way. Up until yesterday, she said she was going to move to France!"
It seems more natural in a surprised response than using "until". Note that in this last example the actions are reversed in order they occurred, and we're emphasizing with "up until" because speaker B is shocked by the conclusion (She decided to go back to university).
And if you said "She kept studying up until dawn", as was previously used as an example. The following question would be "Well, did she get any sleep?"
I've been teaching ESL for 12 years and this had never come across until this week that a student asked me. I was actually pretty puzzled on how to explain it then. But I think it makes sense this way to differentiate them.