Do you have rounded shoulders (kyphosis) or a slouched posture? Do you have a forward head posture? It could be that the way you are lying in bed is compounding this habit.
We spend nearly a third of our life in bed so our sleeping habits can have a profound affect on our whole system.
It can feel comfortable to have a big pile of pillows but this could be largely because they are supporting you in a familiar shape (ie. bent forward). It may not be helping long term though. It is common knowledge that your structure is affected by how you use it. Your muscles, cardiovascular system and bones, etc develop if you exercise, for example. This also applies to the movements you make in your sleep. If these movements are restricted to a bent upper body position by your pillows this affects your structure over time and encourages habits of slumping. Habitually flexing forwards in this way restricts breathing both in your rib cage and neck. It compresses your internal organs. It can be part of back, neck, or shoulder pain pattern.
In the picture above I have a too high head support. You can see how my back is being pushed into a curved flexed position by this arrangement. If I were to habitually sleep in this situation it would affect my structure.
If you wish to change your sleep habits then a good place to start is by simply reducing your pillow height. It may feel mildly uncomfortable or unnatural at first as your body adjusts to the new configuration. If you have far too many pillows it's a good idea to reduce them gradually to give your body time to adjust.
So how high is the right height for you?
Everybody has a different structure to start with and this has to be taken into account. Other health conditions may also affect how many pillows you need and may have to take priority over the following information. If that is not the case for you then a simple way to find a good height is to lie on your back on a reasonably hard surface, e.g. a yoga mat or the carpet and put a pile of books under your head. You need enough books so that your head is neither being pushed forward tucking your chin in, nor being allowed to drop back and so pushing your chin forward and scrunching the back of your neck. (If you're not sure any Alexander Technique teacher would be able to help you find an appropriate height). The book height will then give you a rough guide as to what may be helpful for you as a pillow height at this time.
In the picture above I have a good height of books for me and my type of structure, your book height is likely to be higher than this. A low book height is not something to aim for, you just need a height that is appropriate for you. You can see how this is allowing my shoulders to rest on the support and my back is not being forced into a flexed position.
When you lie on your side you will want a higher support. I recommend purchasing an additional square 65cm x 65 cm pillow so that it can be in bed next to you without taking up too much space. You can then put it under your head on top of your other pillow as you roll onto your side.
How can you get comfortable on a lower pillow?
The muscles that have been holding you in your habitually flexed position now have an opportunity to let go so you can consciously allow them to do this. Let your head really rest on the pillow. It sounds obvious but it can be very easy to accidentally hold onto to your head with your neck and shoulder muscles even when there is a support available for it. Let your back and legs be supported by the bed. Release your stomach muscles, across the front of your shoulders, under and around your rib cage. Allow yourself to expand into the space that this new pillow arrangement has given you. If you find this a challenge you might like to have an Alexander lesson where the teacher can show you how to go about releasing your muscles effectively.