Baby sleep cycles

Understanding the cycles is what made me stop looking for someone to blame. My son’s wakings were not his fault, or mine, or the evening milk’s. They were mechanical.

A baby cycle is 50 minutes, not 90

That is the first number to correct, because the adult value is what you read everywhere.

Age	Cycle length
0 – 3 months	~ 50 min
3 – 6 months	~ 55 min
6 – 12 months	~ 60 min
12 – 24 months	~ 70 min
2 – 3 years	~ 80 min
Adult	~ 90 min

A four-month-old sleeping eleven hours therefore goes through about twelve cycles. Twelve cycle endings, twelve rises to the surface, twelve chances to wake properly.

You do five or six in the same night. They do twice as many.

Before 3 months, the phases do not share a name

This is the point popular diagrams almost always miss, and it changes everything you think you know about a newborn.

A baby under three months does not have light, deep and REM sleep. They have:

• active sleep, the ancestor of REM — they move, grimace, smile, grumble, half-open their eyes, and they are asleep;
• quiet sleep, the equivalent of deep sleep — still, regular breathing, open hands;
• transitional sleep, an in-between also called indeterminate sleep, which makes up around ten per cent of nights and disappears around three months.

And above all: they fall asleep through active sleep, the opposite of an adult. That is why a newborn who has just dropped off squirms and makes noise, and why you think they are awake when they are asleep.

Agité20 min

Calme28 min

20 min — ne pas le poser

vous pouvez le poser

• Sommeil agité
• Sommeil calme
• Sommeil de transition

Around three or four months the adult architecture falls into place. The vocabulary changes, and entering sleep now happens through light sleep.

Léger15 min

Profond37 min

15 min — ne pas le poser

vous pouvez le poser

• Sommeil léger
• Sommeil profond
• Sommeil paradoxal

Deep sleep sits at the start of the night, and that explains a lot

Cycles are not identical from one end of the night to the other. This is the second thing simplified diagrams leave out, and it is the one that answers parents’ real questions.

Deep sleep is heavily concentrated in the first few hours. In a four-month-old, the first cycle can be nearly sixty per cent deep sleep; the last, under ten. REM sleep goes the opposite way: ten per cent at the start, up to forty-five towards morning.

Three very practical consequences.

They are immovable at 10 p.m. and wake at nothing by 5 a.m. Nothing to do with willpower: at 10 p.m. they are at the bottom of their deep sleep, at 5 a.m. they have none left.

Night terrors happen at the start of the night, because they occur in deep sleep. Nightmares come in the early morning in REM sleep. They are not the same phenomenon and are not handled the same way.

The 4 a.m. waking that never ends is not a tantrum. It is the point in the night when sleep is lightest, sleep pressure is nearly spent, and they need the most help to get going again.

The REM share falls with age

A newborn spends about half their sleep time in active sleep. An adult, twenty to twenty-five per cent in REM. The shift happens gradually, with a clear drop around six months.

It is not a defect in quality. That sleep builds the brain, which is exactly why they need so much of it early on. A baby who moves a lot in their sleep does not have poor sleep: they have baby sleep.

What you do with all this

Three practical things come out of it.

A waking between two cycles is normal, including in a child who “sleeps through” — the difference is that they settle back alone. The real question is never “how do I stop the wakings”, it is “can they get going again without me”.

A thirty-minute nap is one complete cycle, not a failed nap.

And a baby who has just fallen asleep is not yet ready to be put down: they have to get through the first phase of the cycle, ten to twenty minutes depending on age, before sleep is deep enough for the transfer.