On nights that are not my turn, I miss him and find it hard to sleep. What practices can calm me?

Make that night alone a night with Allah, so that loneliness turns into calm, not bitterness. Fill the time before sleep with wirid, reading the Qur'an and voluntary prayers, because “in the remembrance of Allah do hearts find rest” (Surah ar-Ra'd 13:28). Pray witir and make supplication for yourself, your husband and your family. Avoid calling your husband repeatedly during your co-wife's turn unless necessary, because that adds to the restlessness and makes things harder for everyone. Set up a calming bedtime routine and keep your phone away. If missing him turns into deep sadness every night, talk to a counsellor. A night filled with worship feels shorter and more blessed. See du'a & prayer.