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Babies who get most of their sleep at night — as opposed to during daytime naps — do better on tests of executive functioning, including impulse control, mental flexibility and working memory, compared with kids who sleep less at night, according to a new longitudinal study.

The findings add to the evidence that early childhood sleep plays a key role in the development of higher-order cognitive processes, which may later contribute to emotional and social adjustment and achievement in school. (More on Time.com: Does Lack of Sleep for Children Mean Obesity?)

For the study, researchers at University of Minnesota and University of Montreal followed 60 infants, with the help of their mothers, from age 12 months to 26 months. At 12-13 months and again at 18 months, mothers were asked to keep infant sleep diaries for three consecutive days, recording their children’s patterns of sleep in half-hour increments over each 24-hour cycle. Based on the diaries, the researchers were able to determine how much total sleep each child got, what percentage of sleep took place at night (between 7 p.m. and 7 a.m), and how fragmented the sleep was (that is, how often the child woke up during the night).

Then, at 18 months and 26 months, the researchers had children participate in various tests of executive functioning. In one such exercise — a test of working memory designed for 18-month-olds — researchers hid a sticker in one of three pots, covered the pots with a blanket and asked the child to remove the blanket and identify the pot that held the sticker. Another test for impulse control designed for 26-month-olds involved the placing of a present under a transparent cup in front of the child. The child was then asked to wait until the experimenter rang a bell — between 5 and 20 seconds — before retrieving the gift.

Researchers also measured the children’s general cognitive functioning at 12, 18 and 26 months.

Compared with children who slept more during the day, those who got more of their sleep at night at 12 and 18 months tended to do better on the executive function tasks at 26 months, especially on those that required impulse control. The greater the proportion of sleep at night, the better kids did. (Neither the amount of total sleep nor number of awakenings at night affected performance, though researchers think the latter factor may have been underestimated, since many babies who wake during night are able to self-soothe and go back to sleep, unnoticed by the parent.) The link between nighttime sleep and higher-order skills persisted, even after the researchers controlled for factors like parents’ income and education and the children’s general cognitive abilities. (More on Time.com: Study: Most Babies Sleep Through the Night (But Not Mine))

“These results raise the possibility that infant sleep affects developing brain structures [associated with higher-order cognitive processes] in the first two years of life,” the authors write, “thereby setting in motion a cascade of neural effects that may carry substantial implications for later executive functioning.”

Many past studies have shown that sleep — or lack thereof — has a profound influence on performance on tests of executive functioning, in both children and adults. It has also been shown that the brain’s prefrontal cortex, which is largely responsible for complex behavior and decision-making, is particularly sensitive to the effects of sleep deprivation. The “restorative features of sleep may be especially prominent in the frontal cortex, which is one of the most active and most densely connected brain regions during wake time, and may thus require more recuperation during sleep,” the authors write.

The study was published in the November/December edition of the journal Child Development.

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