The choice to breastfeed an infant is intensely personal. Yet because of the well established health and psychological benefits of breastfeeding for both the mother and infant, it has also become a medical decision. For this reason, any real or perceived disadvantage of breastfeeding should be carefully evaluated using objective, valid scientific methods.
One of the perceived disadvantages is that the breastfeeding mother will get less sleep at night. This is an important and real concern. Sleep disruption is known to have a major impact on the mother’s ability to function during the daytime and is a risk factor for postpartum depression.
Postpartum women need to consider strategies — with their family and support partners — for ways to maximize their consolidated sleep. However, few scientific studies have addressed whether feeding methods really do affect the mother’s sleep. There are a handful of studies (mostly based on mother’s subjective report) that suggest that breastfed infants sleep longer and awaken less frequently during the night. But this does not tell us whether the mother’s sleep also differs.
Therefore, our goal with this study was to determine whether mothers’ sleep or daytime functioning differed based on whether she was exclusively breastfeeding, formula feeding, or using a combination of the two.
To accomplish this goal, my collaborators Dr. Heather Clawges (a pediatrician and lactation consultant), Ms. Eleanor Santy (an undergraduate research assistant) and I asked mothers to volunteer to wear a wristwatch-type device called an actigraph. The actigraph recorded the mothers’ movements, and told us when each mother was awake and asleep.
Every morning, within a short time after awakening, the mothers used a hand-held computer to tell us her perceptions of her sleep: how often she had awakened that night, how much total time she was awake during the night, and her sleep-quality score. During the day, whenever she fed her baby, the mother also used her hand-held computer to report her current level of sleepiness and fatigue.
The 80 women who participated in the study contributed this information for 15 continuous weeks, starting when their babies were one week old. A researcher visited the mother each week to give her a new wrist monitor and hand-held computer, but did not give her any sleep advice. It is also important to note that each woman was screened before the study started and was not eligible for the study if she had symptoms or a history of depression.
The results of this study were very straightforward: We did not find differences between women who had exclusively breastfed, exclusively formula fed, or used a combination of the two methods on any measure. In other words, feeding method made no difference in terms of objectively measured total sleep time, sleep efficiency (the ratio of sleep to wake during the night), or sleep fragmentation (the amount of sleep interruption during the night) as recorded by the wrist monitor. Nor were there any differences based on the mothers’ subjective reports of their number of nocturnal awakenings, total nocturnal wake time, or sleep quality. Finally, there were also no differences between groups on daytime levels of sleepiness or fatigue.
One sidenote is that we also asked the mothers who gave the nighttime feedings at each week. We were surprised to learn that not one single mother had help giving nighttime feedings, even if she was exclusively formula feeding! The contrast between our findings of no differences based on feeding methods and the previous studies showing that breastfed infants awaken more often at night is intriguing. Common sense would declare that if the infants awaken more often, then their mothers should, too. It is possible that despite their self reports, breastfeeding mothers awaken more often during the night to feed their infants but that they return to sleep more quickly or sleep during feedings, and consequently do not remember these awakenings. We cannot test this unequivocally because the wrist monitor is not sensitive enough to identify discrete awakenings.
Our speculation is that if breastfeeding mothers awaken more often at night, then breastfeeding itself may provide a form of compensation. In other words, breastfeeding mothers may return to sleep more quickly and not remember awakening. This could be because they are not exposed to as much ambient light or physical activity as would be required to prepare formula. It is also possible that breastfeeding mothers are sleeping during feedings. Researchers Qillin and Lee have previously reported that breastfeeding mothers who cosleep sleep more than both those who do not co-sleep and those that formula feed. The major limitation of our work was that we did not collect data about co-sleeping, which is very common in our society despite professional recommendations against doing so.
Breastfeeding may also have a soporific effect. Sanchez and colleagues have shown that nucleotides present in breast milk have both strong maternal circadian rhythms and appear to facilitate a “hypnotic action” in the infant. In addition, differences in circulating prolactin are suspected to have a primary role in sleep architecture differences between breast and formula feeding mothers. Prolactin shows a nocturnal peak, which is vital for milk production, and is usually associated with facilitation of sleep onset and the delta wave brain activity important for restorative sleep.
In sum, our study shows that choosing to formula feed does not equate to improved sleep for the mother. The risks of not breastfeeding should be weighed against the cumulative lack of evidence that breastfeeding has a negative impact on the mother’s sleep.
Breastfeeding photo available from Shutterstock