I have no source to back up my hypothesis, but I suspect it has more to do with a person's activity than with an external time source. In other words, when a person is sleeping, different brain centers and nerve complexes are active than after he's been up and about for most of the day, etc. The same would make sense for the meridians. In an age when most people kept very similar schedules based on the rising and setting sun, it would be natural to assign rather fixed times to these activation periods, but in today's world of electric lights and 24/7/365 lifestyles I wonder if they still hold true.
Yours in Budo,
---Brian---