[J. Tithonus Pednaud]

I can answer that. It's simply because your vocal cords are relaxed after a few hours of inactivity. Vocal cords are basically semi-elastic muscles. High notes are on contraction.

When you have a cold your cords are swollen and unable to contract, thus your voice is usually lower. When you are excited and tense, your voice is higher as the vocal muscles tighten and tense up as well.

Muscles relax and expand during the night. You are taller in the morning as well.