Hello. I Have Been Prescribed The Anti Anxiety Drug Zispin

Hello. I have been prescribed the anti anxiety drug Zispin (mirtazapine) for sleep, in order to help me reduce from Valium 5mg taken nightly for one year through an extremely stressful time. What are the side effects of this drug? Is there another more natural way to improve insomnia, which I have suffered for years, instead of switching dependence from one drug to another? Thanking you for your help, Adrienne