Back muscle spasms during sleep are often caused by muscle strain, poor posture, or sleeping in an awkward position. To relieve them, try stretching your back gently before bed, using a supportive mattress and pillow, and applying a warm compress to the affected area. Staying active during the day and doing strengthening exercises for your back and core muscles can help prevent spasms. If the spasms are severe, frequent, or accompanied by numbness or weakness, it is best to consult a doctor or physiotherapist for proper evaluation and treatment.