Babies feed all day night and day.
They tummies are tiny.
And they "cluster feed" which means they even feed every single hour.
NORMAL.
It is important to nurse/give Formula, when they are hungry. Because, it is according to their... needs. And appetites fluctuates every day.
Feeding rice cereal is not a solution.
And yes, it can cause constipation... because their organs are STILL developing and their digestive system.
For the 1st year of life, per our Pediatrician... breastmilk or Formula is a baby's PRIMARY source of nutrition. NOT solids and not other liquids.
And, solids, is not as nutritionally dense, as breastmilk or Formula. And to give this on-demand, 24/7.
Solids, is not the magic solution, to making a baby sleep all night.
Babies, wake. For hunger or teething or separation anxiety and any needs. Normal.
6 months is also coming up. And that is a growth-spurt period in a baby. Hence, they get hungrier. More often and need more intake (ie: breastmilk or Formula).
Babies do not sleep through the night. Some may, But that is not a guarantee.
No baby or child, has slept the same way all their life. It is not static. Sleep patterns and ability, varies. And per their developmental phases and growth-spurts.
And likewise, no Teen/adult or college kid or elderly, has slept the same way they did as a baby.
Sleep... always changes. And reflects developmental occurrences.
The book "What To Expect The First Year" and "What To Expect: The Toddler Years" is good.
Also a baby should nap.
Over-tired or overly stimulated babies/children, actually sleep worse. Over tiredness, makes a baby have a harder time to fall asleep and have good quality sleep.
For the 1st year for example, my son/daughter would nap 3 times a day. And go to bed at night by 8:00pm.
And they had huge appetites, I nursed, and they woke at night for feedings. Which I did. Even if they were on solids or not.
I primarily did nursing.
After waking in the morning, a baby will typically get tired and need a nap... about 2 hours or 3 hours max, after waking. Even a bath at this age, is an 'activity' for them.
My kids as babies, woke up during the night about every 2-3 hours too.
Normal.