A gentle guide to sleep science support: Cortisol and Cry-It-Out: What the Science Actually Says (Not What Sleep Trainers Tell You)

The cortisol debate around cry-it-out is frequently misrepresented by both sides. Here is what the peer-reviewed research actually shows.

Cortisol and Cry-It-Out: What the Science Actually Says Few topics in baby sleep generate more heated debate than the question of cortisol and cry-it-out. Proponents of cry-it-out claim the research shows it is harmless. Critics claim it causes lasting neurological damage. The truth, as is often the case, is more nuanced than either side acknowledges. This article reviews the actual peer-reviewed research, explains what it does and does not show, and explains why the Gentle Night Method takes the position it does. The Key Study: Middlemiss et al. (2012) [age-appropriate schedule](/sleep-schedule) [regression predictor tool](/regression-predictor) [download free sleep printables](/free-printables) The most cited study in this debate is Middlemiss et al. (2012), published in Early Human Development. The study measured cortisol levels in both mothers and babies during a sleep training program that involved leaving babies to cry. The finding that generated the most attention: after 3 nights of sleep training, babies stopped crying at bedtime but their cortisol levels remained elevated. Meanwhile, their mothers' cortisol levels had synchronised with the babies' reduced crying and were no longer elevated. What this means: the babies had learned to stop signalling distress, but their physiological stress response had not resolved. The mothers, reading the behavioural signal (no crying), believed the babies were no longer stressed. The cortisol data told a different story. What the Pro-CIO Research Shows It is important to be honest about the other side of the research. Price et al. (2012) and Gradisar et al. (2016) found that graduated extinction (a form of cry-it-out) did not produce measurable differences in attachment security, emotional development, or cortisol levels at 12-month follow-up compared to control groups. These studies are methodologically sound and should not be dismissed. They suggest that for average-temperament babies, a period of cry-it-out sleep training does not produce detectable long-term harm. What the Research Does Not Show The pro-CIO research does not show that cry-it-out is harmless for all babies. The studies used average-temperament samples. High-needs babies, babies with anxious attachment, and babies with elevated baseline cortisol are not well-represented in these samples. The research also does not address the cumulative effect of repeated cortisol spikes in sensitive developmental windows. The infant brain is undergoing rapid development in the first year of life, and the long-term effects of repeated cortisol elevation during this period are not fully understood. Why the Gentle Night Method Takes a Precautionary Position Given the current state of the evidence, the Gentle Night Method takes a precautionary position: when a method that avoids cortisol elevation (responsive settling) produces equivalent long-term sleep outcomes to methods that involve cortisol elevation (cry-it-out), there is no reason to choose the m