I started DD1 on rice cereal at 4 months. I plan on doing the same with DD2, who is currently 3.5 months. Even with the WHO recommendation of no solids till 6 months, many pediatricians are comfortable with attempting to start with cereal at 4 months. By the time my DD1 was 6 months, she was well into solids, including cereals and stage 1 fruits and veggies. We never had any problems. Like with most things, you may just want to start and see how it goes. If your LO seems to have tummy problems, or difficulty with the spoon due to tongue thrust, then stop and try again a couple of weeks later.
As far as sleep, as Martina (who is lives in awesome Collingwood notes), for some LO's, the introduction of solids can actually do more harm than good, but those adjustment issues are usually short lived. Also, consider that most babies start STTN between 5-7 months, and even though that is right smack in the middle of the introduction of solids for most people, it does not indicate a cause and effect relationship.