Dropping to one nap: what's actually happening, and when
There is a stretch somewhere after the first birthday where sleep goes strange again, and it catches a lot of parents off guard, because by then you have usually done the hard yards and you are quietly expecting things to just stay smooth for at least a little bit. The naps start fighting you, bedtime drags out, the early mornings creep back in, and most parents I speak to assume that something has gone wrong, or that they have done something wrong. I see this all the time in my consults, this small panic that a baby who finally became a good sleeper has turned into a bad one overnight. And nearly every time, what is actually happening is much less alarming than it feels. Your baby is getting ready to drop their second nap.
When it actually happens
The two to one nap transition usually lands somewhere between twelve and eighteen months, and for most babies it is the back half of that window, closer to fifteen to eighteen months than to a year. The reason the early version is so common online is that one rough patch of naps gets read as readiness, the second nap gets dropped too soon, and then you end up with an overtired baby who is somehow sleeping worse than they were before. So the timing can matter here, but usually just knowing the signs and how to juggle the messy in between of the transition is usually the right call.
Why it looks like a regression
A few things are usually happening around the same time at this age, and that is the part that makes it feel like everything is falling apart at once. The requierments for their sleep pressure is shifting, so the old two-nap schedule that used to fit just does not fit a bigger child anymore, and on top of that there is usually a lot going on developmentally, the walking and the words and the big feelings about you leaving the room. When the schedule and the development collide, you get the short naps, the nap refusal, the split nights, the early wake-ups, all of it at once. It reads as a regression because the sleep gets worse. What it really is, though, is the schedule needing to catch up to a more grown-up brain and body, and that is a very different problem with a very different solution than rewriting every aspect of their sleep.
How to know they're ready, and what to do
The signs are less about one dramatic change and more about a shift in the whole picture. Sleep that was fairly settled starts to go sour, and it can show up anywhere, not just in the naps. Maybe they start taking much longer to fall asleep, for a nap or at bedtime, maybe the sleep itself gets more broken, maybe the second nap starts getting refused or pushed so late it wrecks bedtime. The thing I would watch is the overall pattern across about a week, rather than reacting to any one hard day, because at this age a single rough day tells you very little on its own.
What is happening underneath is that they are starting to need more awake time to build up enough sleep pressure for good sleep, and when their naps do not naturally shorten to make room for that, sleep starts to bump up against itself. So the fix is either to follow their lead if they are genuinely ready for less day sleep, or to gently do it for them by capping or trimming that nap, usually by waking them a bit earlier, and you will often see the overall sleep improve once you do.
The move itself is slow, and the slowness is the bit people tend to rush. You stretch toward a single nap over a week or two, usually toward the middle of the day, and you protect bedtime hard while you do it, because an earlier bedtime is the thing that carries them through the in-between stretch. There will be messy days, and an in-between period where some days look like two naps and some look like one, and that is completely normal, not a sign it is going wrong. If it is too tricky to know what to do on any given day, the approach I usually suggest is simple, one day on and one day off of dropping the nap, so you are easing through it with a plan rather than flip-flopping with none. What you are reading the whole time is the week as a whole, not last night.
It is also worth saying that this is the same pattern I wrote about at the eight to ten month mark, just further along the line. The ages change, the developmental stuff changes, but the shape of it stays the same, a stretch of harder sleep that is actually a sign your baby is moving forward, not backward.
So if your little one is somewhere in this window and the naps have gone sideways, it might be worth sitting with the idea that nothing has broken, and you have not undone your good work. You are most likely just on the edge of a change that was always coming.