My two cents worth.
Your vocal tract is relaxed when you wake up. It tenses throughout the day, due to a number of possible reasons, including, emotional tension, body cycles, bad posture, feelings and mood, type of food ingested, playing guitar, difficult tasks, etc., and gravity.
The vocal tract is attached to all other parts of one’s body, so tension arising from elsewhere can affect the vocal tract, reshaping your sound production methods.
Food, playing guitar, difficult tasks are easy to test, and emotional tension is easy to spot. The others require more awareness–ergonomics, posture, feelings and moods. These latter three can be fixed by ergonomics furniture and posture alignment (this is difficult to do though). The feelings and moods are fixed via posture alignment, as described in meditation principles.
The question is less regarding why you’re tensioning when playing guitar, and instead, why your body doesn’t “detense” after playing the guitar and does so during your sleep. In general, this is probably due to one’s posture alignment isn’t right, because if the posture is correct, then the counteracting muscles will pull and detense the tensed muscles when posture is straight.
>>>Hello, i have a problem with my voice. When i wake up my whole vocal system is very relaxed and i can hit from C2 to G6 and all the notes in between without any strain. My vocal fry is edgy and tone is clear, in short everything is perfect and i am really happy.
The bad news are that this quality and range (i dont care much about my whistle though its fun to have) is lost throughout the day and even within 1-2 hours without doing anything vocally… i mean not vocalizing at all. Vocal fry goes “bye bye” and i have to push to get the very high notes and i am limited from an F2 to a pushed G5 at best or ~G2 – ~D5 “solid” range. Also the quality degrades with little bit air, edge is greatly reduced. Only good thing is my highs E4-C5 become somewhat more powerful and easy to hit with fuller voice.
I think i am tensioning myself. Many times i find out i am tensioning especially when i play guitar or piano difficult exercises i am straining my jaw and probably my neck. This also happens without any activities, i mean doing nothing at all…
Any thoughts / advices?