Sleep, Stress, and Science: Is Cortisol the Enemy?

As a new mum navigating my baby’s first sleep regression, I came across a LOT of information—and, unfortunately, quite a bit of misinformation—about what happens to our babies’ brains when we put them down to sleep in their own sleep space. Much of the chatter is centered around the "cortisol spike" that occurs when a baby experiences discomfort. Ever the teacher, this discord was enough to send me back to school (my safe space), diving into peer-reviewed studies and professional opinions so I could make an informed decision based on science and child psychology, rather than trying to wade through the (often loud, passionate, and absolutely conflicting) advice from my local parenting Facebook groups. And so here, in all its glory, is what I’ve learned about cortisol, babies, and sleep intervention.  

Learning a new skill, such as walking, can lead to short-term cortisol spikes

What Parents Need to Know About Cortisol

Cortisol is a natural and essential hormone (sometimes colloquially referred to as the ‘stress hormone’) that plays a crucial role in regulating our babies’ sleep-wake cycles, metabolism, and immune function. Cortisol follows a daily rhythm, peaking in the morning to promote alertness and declining at night to support sleep. This process begins developing around two months of age and matures by eight months.

If cortisol spikes are normal, why are they so often painted as something to avoid?

It’s important to note that chronic high cortisol (not brief spikes) is a very real concern. Babies with chronically elevated cortisol show:

• 3x higher risk of developmental delays

• Reduced hippocampal growth (which is the part of the brain associated with memory, spatial navigation, emotional regulation, and learning) on MRI

However, there is a big difference between chronic high cortisol and short term spikes in cortisol, which happens for babies during everyday experiences like learning a new skill (we’ve all seen the frustration on their little faces when they try to crawl but faceplant instead), crying, or adjusting to a new routine. Cortisol also naturally spikes when our bodies are getting ready to wake up in the early hours of the morning.

Separating fact from fear

Some research (such as the well known study by Middlemiss et.al.) suggests that cortisol levels in babies undergoing ‘sleep training’ can remain elevated even after they stopped crying; that there is a mismatch between the stress they feel and the demeanour they present. However, all the studies I’ve come across so far have significant limitations. For instance, I haven’t yet seen a study that didn’t rely on strict "cry it out" methods to conduct the research—an approach that isn’t widely favoured today. Before my baby was born, I joined every parenting group I came across in an attempt to learn everything I could (there’s the teacher again!); let me tell you, I have absolutely seen these studies being used to suggest that babies who self-settle have simply learned not to express their distress. I’ve come across quite a few comments like, ‘no amount of sleep for me is worth my baby suffering in silence,’ often left on posts from exhausted parents desperately seeking advice. Every time I read these sentiments, I feel so very sad for those parents reaching out for support, particularly given that we can now refer to more recent studies, with larger sample sizes and more rigorous methodologies, which challenge assumptions that babies who are self settling are quietly experiencing distress.

The case for sleep intervention

• Studies suggest that gentle sleep interventions (e.g., gradual withdrawal methods) may cause lower cortisol spikes compared to chronic overtiredness or daycare transitions.

• Research from Flinders University (2016) suggests that fragmented sleep patterns (often caused by overtiredness) are associated with higher cortisol levels in infants. Importantly, this correlation exists regardless of how babies fall asleep (e.g., independently or with parental assistance).

• Cortisol naturally builds from around 3AM until morning, and is not necessarily linked to routine awakenings during the night. It is, however, more strongly linked to stressful disruptions. A baby who has begun to understand object permanence may wake more often if their sleep environment changes between going to sleep and their first night waking. For example, if they are placed alone in a cot after falling asleep latched or being held.

Extensive research has found that no long-term differences in cortisol levels, emotional health, or parent-child attachment exist between babies supported to sleep more independently and those who aren’t. However, it’s important to note that individual responses vary depending on factors like temperament and parental responsiveness.

The Bottom Line

1. Cortisol rises during night wakings regardless of how a baby falls asleep.

2. Short-term spikes help babies learn to adapt to changes in their environment over time.

3. Chronic sleep deprivation poses a greater risk than temporary stress during gentle sleep support.

Discomfort and stress are inevitabilities in life, but by prioritising consolidated sleep and maintaining responsiveness during awake times, we can foster healthier cortisol patterns - and happier, well-rested babies.