In a New Study, Infants Were Sleeping More at Night Than During the Day 

by 9 Weeks on Average

A new study suggests the 24-hour body clock that regulates waking and sleeping matures over several weeks, starting at about 8 weeks of age. Cortisol, a hormone that helps the body deal with stress, kick-starts a series of subtle physiological changes signaling the onset of adult-like sleep patterns, according to the study, in the January issue of Archives of Disease in Childhood, Fetal & Neonatal Edition. Biological or circadian rhythms in growing fetuses are largely controlled by mothers, but it wasn’t clear when newborns establish their own circadian rhythms, the researchers said.


From 2007 to 2008, researchers in the U.K. monitored 35 healthy, full-term babies from 6 to 18 weeks of age. Every two weeks, the infants’ body temperature was recorded throughout the night and sleep time measured with a device called an actigraph worn around the ankle. Urine samples collected at noon and midnight were analyzed for cortisol and 6-sulfatoxymelatonin (MT6s), a marker of melatonin, a sleep hormone that increases at night. Weekly cheek swabs were collected every six hours over two days and analyzed for circadian gene expression.

A mature pattern of cortisol secretion—indicated by greater hormone secretion during the day than night—occurred at 8.2 weeks on average. This was followed about a week later by the circadian secretion of MT6s. At nine weeks, infants spent more time sleeping at night than during the day.

A mature temperature rhythm, marked by a consistent drop in core temperature in the first few hours of sleep, was seen at 10.8 weeks. Maximum circadian gene expression was recorded at 11 weeks.

This should be interesting information for new parents. Though this was a relatively small number of infants, it indicates that parents shouldn’t expect night sleeping until 2 1/2 months.

— Vimala McClure