The idea that solids will help your baby sleep is an old wives' tale that has been disproven by medical studies. Feeding your baby solids or formula in an attempt to make baby sleep longer is not a good idea for several reasons:
There's no evidence that it will help. Some babies will sleep worse, due to reactions to the formula or solids (tummy ache, etc. are not uncommon), particularly if baby is younger than around 6 months. Two studies have indicated that adding solids to the diet does not cause babies to sleep longer. These studies found no difference in the sleep patterns of babies who received solids before bedtime when compared to babies who were not given solids. Here are the two studies:
Macknin ML, Medendorp SV, Maier MC. Infant sleep and bedtime cereal. Am J Dis Child. 1989 Sep;143(9):1066-8.
Keane V, et al. Do solids help baby sleep through the night? Am J Dis Child 1988; 142: 404-05.