​​Why is our voice deeper in the morning?

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.
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.

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