For most (if not all) people, their voice sounds a lot deeper when they've just woken, compared to how they sound during the day. This effect can easily last for a few hours, like for me today. What are the biological reasons for this? I've heard that it has something to do with relaxation of your muscles during sleep, but I'm not sure how that is supposed to work, and there might be other factors at work.
I'm surprised there hasn't been any answer or comments but yes, this is a fairly well known phenomena. It's quite possible that not everyone finds this to be true for them or possible they've never even noticed it. If a person doesn't need to talk for the first 15 minutes to hour, they might not realise their voice is deeper and/or raspier first thing in the morning.
Speech pathologists, voice teachers and professional singers are certainly aware of this. There's been no solid research on why that I know of. After all who's going to fund research into something that's mainly a curiosity with no medical repercussions or profits to be made? Still, there are various thoughts on why this occurs (outside of acid reflux irritating the vocal cords).
During sleep while we're in a horizontal position, a certain amount of fluid collects in our upper body (read from the neck up), so our vocal cords will swell and thicken slightly. It's why our eyes look more puffy first thing when we arise. During the night, we're not using our vocal cords either but while breathing, air is still passing over them. So while the tissues swell a little, their outer membrane dries a little. To counter the drying effect, the body secretes more mucous as a protective coat. This effect is much worse in people who are mouth breathers.
Another possible reason is what voice teachers and singers think happens, too. Vocal cords, being muscles, need to stretch and contract to vary the voice's pitch. Unused through the night, they're nit as limber and can't stretch as well as later. Vocal cords stretch to reach higher pitches. If that idea sounds far-fetched, just remember opera singers and other professional singers need to limber up before performing. They practice hours each day but before a performance, they need their voice to be at its best. Hence they'll practice singing notes up and down the scale.
Supposedly, drinking a cup of lukewarm water first thing in the morning helps, and tea or coffee is supposed to dry out the throat even more. I never drink coffee but, personally, I've found that first hot cup of tea in the morning doesn't just help my voice return to normal but wakes me up.
I've never heard of this effect lasting for a couple of hours though I haven't gone around quizzing people. :) So try either the lukewarm water or a hot drink and see if it makes a difference. You can also try singing or just talking aloud to limber your cords in the morning.
Edited June 14/17 Links (but not scientific studies) where others have noted this phenomena
Why do people's voices sound deeper in the morning? I gave greater credence to the first answer since it comes from an opera singer.
And from Music SE Why is my voice lower pitched when I get up in the morning
Just a guess - higher pitched sounds require more energy than lower pitched sounds (at the same volume). Just after waking up, you are more likely to be tired, so the same effort to speak receives a lower energy input, requiring either the volume or frequency to decrease.
For reference, my voice actually switches over to low frequencies when I'm tired as a power-saver feature.