In the past three weeks I've gotten over a dozen emails from people with 4-month-olds and 9-month-old asking why their babies are suddenly not sleeping. Which made me realize that I just assumed that everyone who reads here knows about the sleep regressions, but how would we if I haven't mentioned them in too long? So here goes:
At 4 months: Your baby starts going through a developmental spurt that leaves him or her unable to sleep through the night. So even if you've been getting five or more hours in a row, that may go down to two or three hours in a row at night. Once the baby goes through the developmental spurt, the sleep regression will be over.
Naps, though, are a different thing. I don't know where we all get this idea that newborns actually take long naps, but IME it's a dirty lie. Babies seem to be hardwired to take either 20-minute naps or 45-minute naps, until they hit around 5 or 5 1/2 months, at which point the naps stretch out to a better (1-3 hour) length. And you can do all kinds of things to try to get your baby to sleep longer (raise your hand if you've done the hovering-at-the-edge-of-the-crib/cosleeper-to-pop-in-the-pacifier-at-the-20-minute-mark-exactly thing!), but it doesn't work. What does work is just establishing a consistent nap routine, so when your baby finally hits the magical age at which naps happen, you'll be set up to take advantage of it. (and by "nap routine" I mean anything from an elaborate feed-book-rocking routine to saying "time to take a nap, pumpkin" and putting your baby down. Whatever works for the two of you.)
Note: Some babies wil take longer naps from the get-go. Some babies never take naps well. It doesn't mean anything about their future, or your skills. If it seems way out of the norm, investigate. But otherwise, don't stress over it, and take advantage of the opportunities it presents you.
At 8-9 months: Babies hit another developmental spurt and stop sleeping. Their bodies just can't sleep through the night (or however lonng they were sleeping) before. Some kids are very affected at night, some for naps, and some for both. (And some kids of lucky parents aren't affected at all. I hear.)
This is going to last anywhere from a few weeks to a few months. It seems to depend on how your kid slept before. if your kid slept relatively easily before, going down consistently--no matter how you did it--and going back to sleep after wake-ups, then your kid will probably be sleeping fine again by some time around 10 months (I'm guessing 10 1/2 is reasonable). If your kid was not an easy sleeper before, you're going to be in for a longer stretch and your baby may not sleep through until 14-15 months. This is just what I've observed based on the emails and comments about this over the last three years. There's a mini-regression at 13 months, FWIW.
You will probably feel pretty defeated. It seems like 9 months is just about when parents start to feel confident about things, and women start to feel like they may have some semblance of their bodies back (if by "back" you mean that you've noticed that you still have toes and that your limbs mostly work), and the babies are kind of happy and interactive and starting to move around. And then--bam!--your kid's suddenly not sleeping and it seems like it must be something you did and you feel incompetent and defeated.
Personally, I found 9 months to be waaaay harder than 4 months. 4 months just kind of blended in to the first 3 months, in a blur of constant feeding and waking and worry about whether it was all going OK, and 4 months was just slightly bitier than the previous weeks. 9 months, though, knocked me on my ass. I can remember feeling, when my son was 10 months (and about a week away from coming out of that sleep regression), that I was truly the Most Incompetent Mother In The World. And having a serious talk with myself about what, if any, value or self-knowledge I had since the only thing I'd ever known I wanted to be was amother, but now here I was failing so abysmally, so what were my options for the next 50-60 years of my life.
I've gone through some tough times, but nothing--NO-THING--has been the dark night of the soul like 10 months with my first child was.
So, let's share some stories. How did 4 and 9 months make you feel? Anything particularly egregious your baby did, sleeping-wise? How, if you were able to, did you keep your sanity?