...You sleep on your side(s)...on a mattress that has lost its original 'soft' surface and now tends to 'mush out' (as kapok seems to do).
If that is so, then you may only need to get yourself a 1" foam mattress topper to place against the mattress itself.
I then covered this with a light blanket, then the bottom sheet.
I suppose the mattress topper I used was of a medium density of foam. At that time I knew little about foams, and only bought the topper I first found.
The comfort I got was immediate...arm numbness disappeared instantly.
If you try adding a mattress topper, you may need to consider a greater depth to your fitted bottom sheet. See a clerk in a good bedding store for advice, if this is so. Or, use a flat sheet for your bottom sheet.
Recently I read the idea of one person who advocates raising the top of the bed (or mattress) 4" to 8", as an aid to better urination. Apparently the body eliminates more waste materials when the head and torso are raised at an even slope while sleeping.
This person suggested that the whole bed needs to be supported in an even slope, raising every leg at the proper angle...in order not to place any strain on the frame.
This would take the services of a carpenter, I believe.
Another way to do it might be to raise the head of the mattress, itself, with a mattress platform at the angle desired.
I don't know if this change would help an arm numbness problem, but you could check with a chiropractor, and/or a sleep clinic, for other opinions.
One other possible consideration might be posture. Since I have been adjusting mine, without the advice of a professional, I have noticed that even ones posture during sleep seems to matter to some degree.
There are people who specialize in this field...physiotherapists, and practitioners of the Alexander Technique, for instance. See also the CureZone forum for the Dorn therapy. And check with a kinesiologist you trust. A great deal of thought has gone into these methods.
In my opinion, posture only reflects a person's individual inner tensions.
I hope there is something here for you, Olyayeaye, and that you get satisfying sleep very soon.
Fledgling