Middlemiss et al. (2012): Cortisol Asynchrony in Sleep Training
Babies stopped crying, but their stress hormones did not stop
Landmark study measuring cortisol in mother-infant pairs across a 5-day sleep training program. By day 3, babies had stopped crying but their cortisol stayed elevated. The mother-infant cortisol synchrony had broken. The babies were still in distress. They had stopped signaling.
Wendy Middlemiss and colleagues published a 2012 study in Early Human Development that has become the most-cited paper in the critique of extinction sleep training. The team measured cortisol (the primary stress hormone) in mother-infant pairs across a five-day inpatient sleep training program. On day one, when babies cried alone, their cortisol levels and their mothers' cortisol levels were synchronized: high together, low together. By day three, the babies had stopped crying. This was the success metric for the program. But their cortisol levels had not dropped.
The cortisol stayed elevated even though the crying had stopped. The mothers' cortisol, hearing no crying, had dropped to normal. The synchrony was broken. The babies were still in physiological distress. They had simply stopped signaling it. This is the central finding the field has been working with ever since.
5 days
Sleep training program length
Day 1
Mother-infant cortisol synchronized
Day 3
Synchrony broken, baby still elevated
2012
Early Human Development publication
Why it matters
When the success metric for sleep training (the baby stops crying) measures a behavioral change while the underlying physiology stays in distress, the intervention is not teaching the baby to self-soothe. It is teaching the baby to stop signaling.
This is one finding from the research library behind How to Birth a Mother. Everything here traces back to a study, a dataset, or a systematic review.