Does cortisol affect baby or toddler sleep, including through breast milk?

This article is part of a collection inside The Possums Sleep Program called Deeper Dive, which explores the complex scientific, historical and social contexts in which families and their babies or toddlers live and sleep. You don't need to read Deeper Dive articles to be helped by The Possums Sleep Program.

Does your baby or toddler wake a lot because of high cortisol levels?

You might have heard that your baby or toddler is waking a lot at night because of high levels of cortisol, which results from 'overstimulation' during the previous day or evening. Neither is the case. (A study by Tuladher et al 2021 on this topic is discussed at the end of this article.) The problem with this advice is that it can make sleep problems worse, because

• Attempts to keep minimise your small child's sensory motor stimulation (for babies here and toddlers here) will often dial your baby or toddler up and worsen sleep

• Important things that are likely to be causing excessive night waking and sleep problems (babies here and toddlers here) are not addressed

• You might find yourself worrying about causing problems for your little one through your breast milk's cortisol, when you actually don't need to be concerned.

What is cortisol?

Cortisol is not bad for you or your baby, despite what you might hear sometimes. Cortisol is most famous for being a stress hormone, secreted by the kidneys. However, cortisol is actually a necessary and health-giving part of the human body's response to the environment.

Both an adult's and a baby's or small child's stress response system is driven by the sympathetic nervous system (SNS), which interacts with the hypothalamic-pituitary-adrenal axis (HPA). Together, the SNS-HPA triggers the release of cortisol, as well as other neurohormonal signals. In The Possums Sleep Program we refer to the way you help regulate your little one's SNS-HPA system as dialling him down (in babies here and toddlers here).

As parents, we try to keep our little one's SNS-HPA as downregulated as we sensibly can as a pattern over time - knowing we can't always dial babies or toddlers down, no matter what we do, and that this is normal too! Creating a pattern of reasonably dialled down behaviour in our babies or toddlers is an important part of keeping family sleep as easy and no-fuss as possible, night and day.

Researchers measure cortisol levels in the blood stream, in saliva, in hair, and in breast milk. Cortisol levels are highly variable between individuals. There are two main things which influence cortisol levels. These are

1. The body clock

2. Stress levels.

Cortisol levels follow the settings of the body clock

Cortisol does not drive or cause changes in sleep or the body clock setting. Instead, cortisol levels in the blood stream, saliva and breast milk follow or reflect sleep and body clock setting changes. Cortisol helps communicate signals from the circadian clock to the rest of the body. You can find out about the body clock in babies here and toddlers here.

In humans, the master pacemaker, or main body clock in the brain, co-ordinates myriad networks of microscopic and cellular body clocks distributed right throughout the body. You can find out about the body clock in babies here and in toddlers here.

Most tissues and neurohormones, including cortisol, respond cyclically to the master body clock settings. Human cortisol levels show daily, cyclic changes in response to circadian rhythms (or the settings of the body clock). Cortisol levels follow the circadian rhythm in a pulsatile pattern. Generally, cortisol is highest in the early morning and lowest between midnight and three o'clock at night. There is great variability in cortisol levels between people, with normal early morning peaks, for example, varying threefold.

A detectable cortisol circadian rhythm can be measured from as early as two weeks of age in some babies, but only becomes measurable in other normal term babies as late as nine months of age. This highly variable onset of measurable levels mirrors the highly variable body clock settings or sleep patterns of babies. Mid-afternoon cortisol levels are lower than mid-morning cortisol levels in adults. But in babies, once cortisol becomes detectable, there are two cortisol peaks which are 12 hours apart and unrelated to time of the day. This again reflects the immaturity of the baby's circadian rhythm or body clock settings.

Disrupted body clock settings naturally result in measurable changes in many different hormones, including cortisol and also melatonin. You can find out about melatonin in babies here and in toddlers here.

Adapted from Selfridge et al (2015) and Chan et al (2010)

How is cortisol related to stress?

There are three main patterns of cortisol levels which are measured in response to stress.

1. Certain events or experiences place normal stress on the body, resulting in a cortisol peak from which the body quickly recovers.

2. Repeated short-term very stressful experiences result in hyperactivation of the SNS-HPA and persistent high levels of cortisol. There is not time for the cortisol levels to recover back to the levels seen prior to these repeated very stressful experiences.

   • If this chronic SNS-HPA hyperarousal occurs during the very sensitive and highly neuroplastic first weeks and months of a baby's life, it's widely accepted that the chronic hyperarousal of the SNS-HPA interacts with baby's genetic predisposition to alter the settings of the stress response system with life-long effect in a small percentage of these babies. (If your baby is in the first few months of life and cries a lot, you can find out what to do in the When baby cries a lot: brief & simple collection of articles, starting here.)

3. Severe, persisting adverse events are often associated with lower levels of cortisol long-term, due to adaptive compensations in the settings of the SNS-HPA.

If you are stressed, will the higher cortisol levels in your breast milk disrupt your baby's sleep?

You might have heard that your breast milk cortisol causes changes in your baby's sleep or could have bad effects on your baby. This is not true.

• Levels of cortisol in human milk are highly variable, and track or follow the levels of cortisol in the blood stream. But there is no evidence to suggest that higher cortisol levels in your milk affect your small child's sleep or development.

• Your little one's many experiences in family life, including sensory motor nourishment, feeds, and the way you respond to your baby or toddler, will have far greater effect on your small child's developing stress response system and cortisol levels, than the small and variable amounts of cortisol in your breast milk ever could.

So much in life is outside our control. Some little ones will inevitably experience more stress and adversity than others. Whilst I wish for, and do my best to contribute in some small way towards, a world where every baby or toddler has equal opportunities and is protected from severe adversity, there are many little ones around the planet who will make their way through life having had very early and profound experiences of trauma. I join those many voices calling for societal change, and for a world that always prioritises the wellbeing of children.

In the meantime, the changing levels of cortisol which have been observed in toddlers' saliva or hair in relation to their sleep patterns, or the changing levels of cortisol measured in a mother's milk, do not affect our babies' and toddlers' developmental outcomes. The belief that cortisol levels affect our small children's wellbeing and sleep causes unnecessary worry and stress for families.

An influential study on cortisol and infant sleep by Tuladher et al 2021 confuses association with causation

In an influential 2021 study conducted in the USA, which has contributed to the mistaken belief that high levels of cortisol result from overstimulation, Tuladher et al investigated the 12 month old children of 90 predominantly white, highly educated women, 60% of whose babies had breastfed in the previous three months. The authors used actigraphy, and salivary and hair cortisol measures.

This study has multiple limitations. Importantly, it fails to take into account the dynamic and contextual nature of the body clock settings as a pattern over time, and the dominant role played by the body clock in affecting cortisol levels. Circadian clock settings respond before all else to light, and are also shaped by multiple behavioural and environmental factors over time, which were not considered in this study.

Though the authors acknowledged that they could not conclude causality, they nevertheless confused association with causation in the interpretation of their findings. For example, the authors found that later parent-reported night-time sleep onset and shorter average night-time sleep duration were associated with higher hair cortisol readings. They found that total cortisol exposure across a given day predicted duration of infant sleep for that night. They found that those with higher evening cortisol levels fell asleep later that same night. The authors concluded: "Bedtime routines would calm the infants and lower their cortisol levels ... The current study underscores the value of setting an early bedtime for infants. ...Taking measures to maintain healthy cortisol levels throughout the day and in the evening and ensuring earlier sleep onset may, therefore, be important in reducing infants’ chronic exposure to cortisol. This, in turn, will decrease infants’ allostatic load, ultimately diminishing their risks of long-term health problems."

Yet we don't know, for example, if these parents were worrying about their babies shorter sleep durations and later bedtimes (which is a widespread concern amongst many affluent white US parents and may have been a reason why they were willing to be recruited for this study). We don't know if those parents were attempting sleep training approaches, which dial up both parents and babies, and which elevate cortisol levels.

Or, to give another example, higher evening cortisol levels may simply show that the afternoon cortisol decline is still underway, and that the phase is set later than it is for those who go to sleep earlier. Although higher total levels of cortisol were found, this could be due to cumulative and normal effect of increased waking hours.

Given the many unknowns in the implications of cortisol level measurements, these findings cannot be seen as pointing to the possibility of abnormal allostatic loads over time with potential developmental impacts. In summary, the study did not take into account 1. the unknown impact of now high interindividual variation in daily sleep durations on cortisol measurements; 2. the impact of different circadian clock settings on levels of cortisol; 3. the impact of normalised societal information about sleep and sleep durations, which increases parental stress and may dial babies up.

Recommended resources

https://theconversation.com/no-you-cant-blame-all-your-health-issues-on-high-cortisol-heres-how-the-hormone-works-203162

Selected refeerences

Egeli TU, Tufekci KU, Ural C. A new perspective on the pathogenesis of infantile colic: is infantile colic a biorhythm disorder? Journal of Pediatric Gastroenterology and Nutrition. 2023;77(2):171-177.

El-Farhan N, Rees DA, Evans C. Measuring cortisol in serum, urine and saliva - are our assays good enough? Annals of Clinical Biochemistry 2017;54(3):308-322.

Hechler C, Beijers R, Riksen-Walraven JM, De Weerth C. Are cortisol concentrations in human breast milk associated with infant crying? Developmental Psychobiology. 2017;60:639-650.

Khoury JE, Ahtam B, Ou Y, Jenkins E. Linking maternal disrupted interaction and infant limbic volumes: the role of infant cortisol output. Psychoneuroendocrinology. 2023;158:106379.

Morris MC, Hellman N, Abelson J, Rao U. Cortisol, heart rate, and blood pressure as early markers of PTSD risk: a systematic review and meta-analysis. Clinical Psychology Review. 2017;49:79-91.

Tuladhar C, Schwartz S, St. John AM, Meye JS. Infant diurnal cortisol predicts sleep. Journal of Sleep Research. 2021;30:e13357.

Vacaru SV, Brett BE, Eckermann H, De Weerth C. Determinants of maternal breast milk cortisol increase: examining dispositional and situational factors. Psychoneuroendocrinology 2023;158(106385).

Wong SD, Wright KP, Spencer RL. Development of the circadian system in early life: maternal and environmental factors. Journal of Physiological Anthropology. 2022;41(22):https://doi.org/10.1186/s40101-40022-00294-40100.