How can I keep my nice low bass voice I have in the early morning hours throughout the entire day?
Full resonant low notes come when the vocal cords are not stretched, so you will have to see if you are taxing the voice and stretching the cords when you speak or sing higher. If so, you will need to develop good breath support in conjunction with a stable larynx and a very precise, unpressured phonation. You will also want to have some head resonance blended in with the full resonance of the chest.
Another element is that as we call our spirits back from dreaming and into our bodies, we become energized and active. As the voice rises and gathers resonance, it stimulates the brain, the organs and glands. It also stimulates your listeners’ brains, organs and glands. So you would not want a sleepy morning voice all day. I recommend falling in love with all the capacities of your voice — it is awesome to have an entire rainbow of colors to express with rather than just one, low morning quality. Base your idea of what your low notes are on your range when the voice is fully warmed up.