According to the Riley Children's Hospital, it is a myth that more solid food will help baby's to sleep better:
Child Rearing Myth:
Adding rice cereal to your baby’s diet will help your baby sleep through the night.
Child Rearing Fact:
Hunger is not waking your baby, and rice cereal does not help your baby sleep through the night. Your baby is a light sleeper and once awakened, can’t get back to sleep without help. When your baby develops a mature sleep pattern and learns to self-comfort when awake but not hungry, you’ll get a good night’s sleep.