Thank you for your question on ehealthforum.com
Numbness of a limb ( either arm or leg) can happen when slept on side due to compression of sensory nerve supplying the leg or arm. This is especially common in people with back problems due to vertebral column misalignment, disc changes.
So, naturally the underlying problem needs to be attended. There are no quick fix remedies for this, but it is helpful to give soft support(like pillows) to the back when sleeping.
An opinion from orthopaedician and physiotherapist is very helpful in this regard. Thank you.