Neck curvature

Hello,

I have a question about changing the curvature of the neck.  I have been sleeping with cervical pillows for years and when I lie on a flat surface, there is a space between the floor and my neck that I can slip my fingers into--my neck does not lie flat along the floor.  If I understand correctly from reading your book, it should be flat or almost flat.    I have tried using several regular pillows of varying thickness, and when I wake up I find that I feel a bit stiff in the neck/shoulder area, and also sometimes in the sacral area.    I assume that over time I will adjust to the pillows, but I was curious if you have observed a general length of time that it takes for the neck to adjust to the new pillow and to lose its excess curvature, if one is using a good pillow and practicing good posture.  Thanks!