Cortisol（皮质醇） may be the Swiss Army knife of hormones¹ in the human body-just when scientists think they understand what it does, another function pops up. While many of these functions are understood for adults, much less is known about how cortisol operates in babies and toddlers（学步儿童） , especially when it comes to an important phenomenon called the cortisol awakening² response, or CAR. For the first time, psychology³ researchers from the University of Georgia Franklin College of Arts and Sciences have shown that this response for infants is opposite of what it is for adults. The new information could have implications for how infants handle stress and why proper care from their mothers could affect how growing children react to cortisol in later life.

"Surprisingly, the CAR hasn't been widely studied in infants or young children," said psychology doctoral student Melissa Bright, who led the study. "There is consensus⁴ that the adult pattern of cortisol response isn't present at birth, but much less is known about when in the first year of life it is established."

Other authors of the research, just published online in the journal Developmental Psychobiology, are Janet Frick, a faculty⁵ member in UGA's department of psychology and director of the UGA Infant Research Lab, and Douglas Granger of Johns Hopkins School of Nursing.

Cortisol is produced in the adrenal gland⁶（肾上腺） and has dozens of jobs in the body. It is released in response to stress and can increase blood sugar, suppress the immune system and aid in metabolism⁷. And yet it works in vastly more arenas⁸ than those. One such area is the CAR, and when adults awake, an organ "team" called the hypothalamic-pituitary-adrenal axis⁹（轴） works together to release cortisol-somewhat as a sentinel-alerting the body to stress or threats.

The new study discovered that in infants, cortisol levels actually didn't increase but stayed stable from the time they awoke in the morning and for half an hour afterward¹⁰. Cortisol levels didn't change following naps, either. Interestingly, the research team also found a mother-infant cortisol association called "psychological attunement," confirming recent research that cortisol levels between mothers and infants are correlated.

"Taken together, these findings raise an interesting possibility," said Frick. "In infancy¹¹, cortisol responses may be less dependent on hard-wired biological rhythms and more influenced by the HPA axis activity of the baby's immediate¹² caregivers."

The team conducted the research using 32 baby-mother pairs. Nineteen of the babies were female and 13 were males, and they ranged in age from 7.8 to 17.4 months. After agreeing to participate, mothers were instructed to collect saliva¹³（唾液） samples using cotton swabs from inside the mouths of their infants and then themselves four times on a single day: when the infants awoke in the morning, 30-45 minutes after the baby awoke, when the baby awoke from its first nap of the day and 30-45 minutes after that. Other requirements applied¹⁴, but they weren't difficult for the mothers to follow, said Bright.

No one knows at precisely¹⁵ what age the CAR begins in humans, though it had previously¹⁶ been predicted to be present sometime in the first year. The current data indicates that it emerges at a much older age, however. Why babies don't emit rising amounts of cortisol in response to awakening isn't clear, either.