SInce you are breastfeeding... wait on cereal - until 7-8 months. I say this because the GI tract isd open and immature and will stay that way until about 7 months old. If you introduce foreign foods now, you are also introducing harmful bacterias that will inflame and irritate her senesitive GI lining - possibly cause severe bm extremes (gassy, diarrhea or harsh constipation.
5 months old!! Her tongue and gag reflex aren't fully capable of swallowing textures of any kind other than pure liquids. If she aspirates, then she can get pneumonia or seriously harm herself.
Starting cereal will not make her sleep anymore than she does already.