Brief Answer:
several possibilities can be there..

Detailed Answer:
Hello XXXXXXX Welcome back to HCM..

I've gone through your query and understand your concern..

Now since you're experiencing this problem during sleeping only, so the first reason to consider is the sleeping posture..please go through the following questions and answer them so that I can guide you more specifically..

Does it involve both of your hands ? or any particular hand ?

Can you please try to locate which part of the hand it's affecting the most(upper/lower/above wrist/below wrist or even above that) ?

Have you noticed the area in light while this had happened ? any discoloration (i.e.whitish/bluish) over the area ?

Is there any pins/needles sensation over the area ?

Any change of temperature (mostly cold) over the area ?

What worsens/improves this ?

Also you should note that with a past history of breast carcinoma and recent recurrence, and radiotherapy, has it's own set of side effects, most noticeably impaired blood circulation, neuro-muscular degeneration and several others..

Other possibilities can be impaired blood supply due to postural variations, Vit-B12 and electrolyte deficiency(calcium and magnesium most noticeably)-however it's unlikely since you've mentioned it's only happening during sleep,

Since it's bothering you for past 3 months, my advice would be to get yourself clinically evaluated by your treating Doctor as there is a need to get physically examined for this..

Let me know any further queries, i'll be glad to clarify..

Take Care
Kind Regards

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