I haven't exactly got a lot of personal data from fasting, but this is for sure:
1) When I first tried fasting, I ate 600 cals, 14g carbs in the morning, and about 12 hours later, my bg was down to 3.9 mmol/l, which is the lowest I've ever seen. Checked 3 times in total, to confirm. I woke up the next day at 3.9 also - lowest morning reading I've ever seen, and also this confirms that I rarely if ever get the dawn phenomenon.
2) I've gone without food for 12 hours regularly, sometimes deliberately not eating anything after about 6:30pm, and breakfast is about 7:30am, so that I am doing a sort of 12 hour fast while sleeping. My bg has never got down near 3.9 doing this, not by the evening or morning. Again, I don't think dawn phenomenon explains this - my morning bg has never been higher than my bedtime bg.