We started solids around 5 months. Things like avocado or banana pureed with breastmilk to make it really thin. We started at about 3 times a week and increased slowly. One of my twins was really into solids and the other didn't care as much. She still doesn't really eat much (16 months), but somehow both twins are exactly the same size. Honestly, solids didn't help with sleeping, so you might have to try some other solutions for that.