Waking up early... My earliest symptom again.

Aislin
With my first pregnancy in May (ended in a miscarriage but I'm sure this wasn't an indicator) every day from about three weeks pregnant (7dpo) I woke up between 4am and 5am, without fail. Before I knew I was pregnant, it was my first symptom.
I remember posting about it at the time and a lot of women saying they were in the same boat (most of whom went on to have healthy babies) I'm now about 4 weeks pregnant (woohoo! Finally!) and sure enough every morning for about a week 
So I was wondering if there's any scientific reason this happens? Is there any logic to waking up at the same very early time every day? (Example: does hcg affect the circadian rhythm? Does the egg multiply quicker at night? Etc Etc)