The study, conducted by Monell Chemical Senses Center, reports that the presence of nicotine in the breast milk of lactating mothers who smoke cigarettes upsets their infants' sleep patterns.
"Infants spent less time sleeping overall and woke up from naps sooner when their mothers smoked prior to breastfeeding," says lead author Julie A. Mennella, PhD, a psychobiologist at Monell.
The findings, published in the journal Pediatrics, raise new queries regarding whether nicotine exposure through breast milk affects infant development. While many women quit or reduce on smoking while pregnant, they often relapse following the birth of the baby.
"Because nicotine is not contraindicated during lactation, mothers may believe that smoking while breastfeeding will not harm their child as long as the child is not exposed to passive smoke. However, there has been very little research on either short- or long-term effects of nicotine delivered through breast milk," Mennella says.
To investigate, Mennella and her team recruited 15 mother-infant pairs. All of the women had smoked before and during pregnancy, although they cut down while they were pregnant. At the time of testing, the women averaged about 10 cigarettes a day, but the amount smoked ranged from 1 cigarette to 30 cigarettes daily.
Each mother smoked one to three cigarettes immediately prior to the observation period on one day and refrained from smoking on the other. On both occasions, mothers breastfed their infants on demand over the ensuing 3-1/2 hours. Following each feed, mothers laid infants down on their backs in a crib or on the floor. Levels of nicotine and cotinine, a major metabolite of nicotine, were measured in breast milk samples provided by the mothers before each feed, allowing researchers to determine the dose of nicotine passed to each infant.
The results showed that total sleep time over the 3-1/2 hours declined from an average of 84 minutes when mothers refrained from smoking to 53 minutes on the day they did smoke, a 37 percent reduction in infant sleep time. This was due to a shortening of the longest sleep bout, or nap, and to reductions in the amount of time spent in both active and quiet sleep.
The level of sleep disruption was directly related to the dose of nicotine infants received from their mothers' milk, consistent with a role for nicotine in causing the sleep disruptions. Infants consumed the same amount of breast milk on both days, suggesting that they were accepting of tobacco flavor in breast milk.
Noting that children whose mothers smoke are more likely to smoke as teenagers, Mennella speculates that early experiences with tobacco flavour during breastfeeding may increase its appeal later in life.
The present findings highlight the need for targeted smoking cessation programs that address issues relevant to lactating women. Mennella suggests that concerns about tobacco flavor in their milk and disruptions of their infants' sleep may help motivate breastfeeding mothers to abstain from smoking.