Babies with higher stress hormone levels late in their mother’s pregnancy can end up having trouble falling asleep, researchers say.

Sleep research suggests that measuring cortisol levels during the third trimester can predict infant sleep patterns up to seven months after a baby’s birth.

Babies often wake up in the middle of the night and have trouble falling asleep.

A team from the University of Denver says one possible but unexplored reason for this is how well the baby’s hypothalamic-adrenal-pituitary (HPA) system is working. 

The HPA system is well-known for regulating the stress response and has previously been linked with sleep disorders when it’s not working optimally. Cortisol is the end product produced from the HPA axis.

Cortisol is a steroid hormone produced by the adrenal glands found on top of each kidney. It plays a crucial role in several body functions, including:

Regulation of metabolism: Cortisol helps regulate the metabolism of proteins, fats, and carbohydrates, releasing energy and managing how the body uses these macronutrients.

Stress response: Often referred to as the “stress hormone,” cortisol is released in response to stress and low blood-glucose concentration. It helps the body manage and cope with stress by altering immune system responses and suppressing non-essential functions in a fight-or-flight situation.

Anti-inflammatory effects: Cortisol has powerful anti-inflammatory capabilities, helping to reduce inflammation and assist in healing.

Blood pressure regulation: It helps in maintaining blood pressure and cardiovascular function.

Circadian rhythm influence: Cortisol levels fluctuate throughout the day, typically peaking in the morning and gradually falling to their lowest level at night.

Collecting hair samples is one way to measure fetal cortisol levels in the final trimester of pregnancy.

“Although increases in cortisol across pregnancy are normal and important for preparing the fetus for birth, our findings suggest that higher cortisol levels during late pregnancy could predict the infant having trouble falling asleep,” says lead co-author Melissa Nevarez-Brewster in a media release. “We are excited to conduct future studies to better understand this connection.”

The team collected hair cortisol samples from 70 infants during the first few days after birth. Approximately 57 per cent of the infants were girls. When each child was seven months old, parents completed a sleep questionnaire including questions such as how long it took on average for the children to fall asleep, how long babies stayed awake at night, and times the infants woke up in the middle of the night. The researchers also collected data on each infant’s gestational age at birth and their family’s income.

Babies with higher hair cortisol levels in late pregnancy took longer to fall asleep at seven months than those with lower cortisol levels. According to the authors, these findings will serve as a key basis for studying the long-term effects of fetal cortisol production on sleep health during infancy and childhood.

More specifically, more research could explain how babies shift into different phases of sleep and how much “sleep pressure” they accumulate throughout the day, which affects how fast they fall asleep at night.

“The results indicate that there may be prenatal influences on sleep health early in life, pointing to the need to better understand what factors may set the stage for better sleep health in infancy and beyond,” says Nevarez-Brewster.

The findings are published in the journal SLEEP.