What Are The Side Effects Of Long Time Usage Of Melatonin Tablets?

My mother-in-law is 90 years old and requires 24/7 care from my husband and myself daily. Recently, she has been very restless, sleepless, and aggressive in the evenings and throughout the night. Sundowner's Syndrome has been discussed with us as well as the use of Melatonin for her at night. Are there any side effects we should be aware of? Thank you so much.