In Russian and Bulgarian,
Sanya (Ñàíÿ) and
Sonya (Ñîíÿ) are different and unrelated pet names, both female.
Sanya comes from
Alexandra, and
Sonya comes from
Sofia.
Sonya and/or
Sanya coming from the Slavic word for "sleep" is a naive etymology, not possible in Bulgaria, not probable in Russian (everybody in Russia knows that
Sonya is a pet form of
Sofia). This confusion could appear in former Yougoslavia only.