Grand Vulcana

What actually changed

The four-month regression doesn't mean your baby has lost a skill. It means sleep has been rebuilt.

Newborn sleep has two stages, active and quiet, and babies drop into it through REM. What replaces it has four: light, deep, REM and a transitional stage, entered through non-REM the way adults do. The cycles get organized, and they stay much shorter than an adult's.

The cost is at the joins. A newborn sleeps through the transitions between cycles. A more mature baby notices them, so every cycle boundary becomes a possible waking.

Underneath, several things arrive at once. Sleep spindles, the brain waves tied to learning and memory, appear around two to three months and become more organized through four to six. The body clock responds more strongly to light and dark. The cortex takes a larger part in regulating sleep, and the systems handling sleep-wake transitions mature.

Rolling changes the rules

Rolling is the motor milestone of this stage, and it changes the safety rules completely.

A study in JAMA Pediatrics found that sleep position influences when babies reach rolling milestones. Once they can roll independently, they should be allowed to find their own comfortable sleep position.

Instead of a swaddle, a wearable sleep sack gives warmth with the arms free, and an arms-free transition sack lets you take one arm out at a time.

Longer days, fewer naps

Wake windows stretch across these months: about 1.5 to 2 hours at four months, 1.75 to 2.25 at five, and 2 to 2.5 at six. The nervous system is becoming more regulated and less easily overwhelmed, so staying awake costs less than it did. Ranges exist for a reason, and some babies need longer windows, particularly nearer six months.

Naps come down with it, from four or five to three, and not in a straight line. At four months there are usually four, with the last one short and unreliable. At five it's three some days and four on others, and both are fine. By six, most babies have three: morning, midday, afternoon.

The signs the fourth nap is going: they fight it consistently, or take a long time to fall asleep for it. It pushes bedtime very late, or they seem content with a longer morning window.

Nights start pulling together

The nights consolidate even while the days feel like chaos. At four months, two to four wakings and 9 to 11 hours in total. At five, two to three wakings and 10 to 11 hours. By six, one to three wakings and 10 to 12 hours.

Bedtime settles into a steadier point each evening. The first stretch of the night is usually the longest, often four to six hours. And night feeds may start dropping away, particularly where weight gain is going well.

They notice you leave now

If your five-month-old cries when you leave the room after the routine, that's development rather than a habit you created.

Between four and six months their thinking expands quickly. They work out cause and effect, including that crying brings you back. They start anticipating what comes next in a familiar sequence, and noticing when something changes. Early object permanence begins, though the full version, understanding that you still exist when out of sight, develops more around eight to 12 months.

So routines matter more than they did, because your baby expects them now, and inconsistency is more confusing than it was.

Separation anxiety proper usually arrives around eight to 12 months and may peak between 12 and 18. What shows up here is the first glimmer. Recognition of familiar faces sharpens, and they show a clear preference for the people who care for them. Early distress at separations may appear at bedtime, and comfort objects (loveys, blankets) may begin to hold meaning. At this age that happens outside the crib: loose objects stay out of the sleep space until after the first birthday.

Peek-a-boo helps, because it rehearses you coming back. So does a goodbye that happens the same way each time, something of yours with your scent during the day, and a routine they can predict.

Why this one is so hard

It's intense, and it's meant to be. Most parents here feel some shock that a baby who was sleeping well suddenly isn't. Confusion, as wakings spike and short naps get shorter. Exhaustion arriving just when things had started to feel better. And a worry that something is wrong, which it isn't.

Four things overlap, which is why it lands so hard. The architecture change touches every sleep, day and night. The swaddle goes, taking the one tool that reliably worked. The cognitive leaps make your baby more aware and more demanding. And all of it arrives just as you were starting to feel competent.

Around six months, as the naps settle into three and the architecture stops shifting, this eases.