Increased bursts of sleep among infants are linked to infant growth spurts in body length, a study published in the journal Sleep found.
Instead of relying on parental recall of infant sleep patterns and growth, the study recorded real time data over a duration of a four to 17-month span. Daily sleep patterns of infants were recorded by 23 parents involved in the study, and 5,798 daily recordings were analyzed. The study included fourteen girls and nine boys, with a median age of 12 days, who were all healthy at birth and free of colic or medical complications during their first year.
Mothers were asked to keep daily diaries detailing sleep onset and awakening, as well as noting whether their babies were breastfeeding, formula feeding, or both, and whether their infant showed signs of illness, such as vomiting, diarrhea, fever or rash.
Growth in body length was assessed using the maximum stretch technique, performed semi-weekly for 18 infants, daily for three infants and weekly for two infants. The monitoring lasted between four to seventeen continuous months.
The results revealed that infants had irregular bursts of sleep, with 24-hour duration increasing at irregular intervals by an average of 4.5 hours a day for two days.
Sleep episodes per day also increased in intermittent bursts of an average of three extra naps per day for two days, the study showed.
“These peaks in total daily sleep duration and number of sleep episodes were significantly associated with measurable growth spurts in body length, which tended to occur within 48 hours of the recorded bursts of sleep,” the study says.
The probability of a growth spurt increased by a median of 43% for every additional sleep episode and 29% for each additional hour of sleep, the study found after further analysis.
“The results demonstrate empirically that growth spurts not only occur during sleep but are significantly influenced by sleep,” says principal investigator and lead author Dr. Michelle Lampl, Samuel Candler Dobbs Professor in the department of anthropology at Emory University in Atlanta, Ga.
“Longer sleep corresponds with greater growth in body length.”
In addition, the study showed that the sex of the baby made a difference in sleep patterns relating to growth.
“Growth spurts were associated with increased sleep bout duration in boys compared with girls and increased number of sleep bouts in girls compared with boys,” says Lampl.
Boys, in general, had more sleep bouts and shorter sleep bouts than girls, but there were no significant effects on total daily sleep for neither the sex of the infant nor breastfeeding, the study says.
However, the study found that breastfeeding, as opposed to formula feeding, was associated with more and shorter sleep bouts.
Parents who can easily become frustrated with the varying and unpredictable infant sleep patterns can be comforted by the results, Lampl adds.
“Sleep irregularities can be distressing to parents,” says Lampl. “However, these findings give babies a voice that helps parents understand them and show that seemingly erratic sleep behavior is a normal part of development. Babies really aren’t trying to be difficult.”
The exact nature of the relationship between sleep biology and bone growth is still unclear, according to Lampl and co-author Michael Johnson, PhD, professor of pharmacology in the University of Virginia Health System.
But they do know that the secretion of growth hormone is known to increase after sleep onset and during the stage of slow wave sleep, which could help to stimulate bone growth. These hormonal signals could help support anecdotal reports of “growing pains,” the aching limbs that can wake children at night.
In some cases, Lampl and Johnson speculate that other parts of the body could be growing. The research also found that longer sleep bouts in both girls and boys predicted an increase in their weight and body-fat composition that ties to an increase in length, implying an anabolic process, or growth.
Even with a statistically significant link between bursts of sleep and growth spurts, no conclusive evidence was found. The study said that some sleep alterations occurred without a growth spurt, and that not every spurt was preceded by a burst of sleep.
An example in another new study being published this month found that infant head circumference grows in intermittent, episodic spurts, suggesting that sleep may be only one component of an integrated, physiological system that underlies growth timing.
“It opens another door to understanding why we sleep,” she said. “We now know that sleep is a contributing factor to growth spurts at the biological level.”
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