we're only at 4 months but this is guiding us through the sleep thing
tldr is that assuming a healthy, fed, clean diaper child - all other issues you're having with your kid is sleep related (vs just random behavior/colic ect). He wants your kids on a sleep schedule asap - we now have our 4 month old sleeping through the night 12 hours (got lucky with this) and taking a 2-3 hour morning nap and a 2-3 hour afternoon nap. 630-7pm go to sleep time is important and not taking naps will result in a sleep deficit that will result in evening sleep getting all fucked up.
extra tldr not getting enough sleep results in not being able to sleep, getting more sleep results in kid wanting to sleep more
sleep begets sleep, apparently it's near impossible for a child to get too much sleep