Does anyone know why certain voices can relax someone to the point they fall asleep?

I recently read a question on fluther about a person who can’t sleep. It got me wondering why is it that certain voices can knock me out. I use to watch 20/20 but two of the anchors on there can KO me in a matter of minutes. Even if I record it and watch it during the day. Its like being in a hypnotic trance. Anyhow, I was wondering why is it that these two anchors who aren’t boring in the least, can KO me in a few short minutes. It got so bad at one point that I couldn’t keep my eyes open for longer than a minute.
I guess what I am hoping for is someone to explain the mechanics involved.
How does the process happen? I know you relax with certain voices but what exactly happens following the relaxing stage and how is it possible to relax so fast?