Question about vocal ranges

[xdrummer2000]

In the past year or so I started gaining a lot of respect for singers such as J.D. Sumner from the Stamps Quartet and Richard Sterban from The Oakridge Boys due to their insanely low vocal notes. Heck, J.D. once held the world record for a double low c (lowest C on a standard piano).

Needless to say, I became inspired to try and lower my vocal range (not to such extreme levels as their's, though--I feel I'm 100% incapable of getting anywhere near their low notes). I don't want to be a singer or anything (trust me, I DO NOT have a good voice at all), it's more of a fun thing and a personal goal. It may seem useless, but it's still something I want to try and do. My goal is to eventually get down to low A (an octave above the lowest A on the piano). So far I've lowered my range a little bit, but it's taken me months and months.

Anyway, I discovered something weird one day while I was trying to do this--I found that in the mornings when I first wake up, I could sing around one or two notes lower than I normally could. I've asked a few people about this who can sing really well, but they told me they weren't sure. I'm hoping that some of you guys on here could answer my question.

I'm just wondering...why is it that I can sing lower in the morning than in the evening or nighttime?