The myth of the 'good sleeper' baby

Every consult I have ever done that includes a mother of more than one child eventually arrives at the same sentence. "My first slept through at 8 weeks. I thought I was just good at it. Then I had my second."

The myth of the good sleeper baby is responsible for more parental shame than almost anything else I deal with. So let me say it clearly: your baby's sleep is not a referendum on your parenting. It is a combination of biology, temperament, environment, and timing.

What the research says about variation

Large longitudinal studies of infant sleep (including the ALSPAC cohort, which tracked over 14,000 children) show that by 6 months, about 38% were "sleeping through" (defined as a 5-hour unbroken stretch from 10pm). By 12 months it was 65%. By 18 months it was 80%. The rest? Still waking, in homes where the parents were doing nothing wrong.

This means around 35% of babies at the end of the first year are still waking at least once a night. That is not a small minority. That is the second-largest group in the study.

Temperament is real and largely heritable

Mary Rothbart's foundational temperament work, refined through the Infant Behavior Questionnaire, identifies three broad infant temperament clusters: easy, slow-to-warm, and difficult. Roughly 40% of babies fall into the easy cluster. They sleep more easily, eat more predictably, and self-regulate faster. They are the ones whose mothers say "I am just a really chill person, my baby picks up on it."

Roughly 15% are difficult-cluster infants. High intensity, slower to settle, more sensitive to environmental changes. The rest are somewhere in between.

Twin studies show temperament heritability around 50%. Half of your baby's sleep tendency is in their genes. The other half is shaped by everything you do. The split is important: you have meaningful influence, but you are not starting from a blank slate.

What this means in practice

If your sister-in-law's baby slept through at 8 weeks and yours did not, the most likely explanation is temperament plus a bit of luck. If your first slept through at 8 weeks and your second is still waking at 9 months, the most likely explanation is the same: different temperament, different baby.

The corollary: every baby can learn good sleep skills, but the starting points are different and the timelines are different. A difficult-cluster baby with good sleep skills will out-sleep an easy-cluster baby with poor sleep skills. Skills are the leverage point. Temperament is not.

The intervention question

Should you do nothing if 35% of babies do not sleep through at 1 year naturally? No. The studies on naturally non-sleeping infants find lower parental mental health scores, more inconsistent caregiving, and more downstream issues for the parents. Even if your baby is in the 35%, working on sleep is for your wellbeing as much as theirs.

What I want you to stop doing is comparing. The baby in the next room and the baby in your sister-in-law's house are different infants. Their sleep curves are not parenting league tables. Pick the right plan for the baby you have, run it consistently, and let the comparisons go.