Mothers who use a breast pump to express milk during the day and then bottle-feed it to their baby at night may be letting themselves in for a sleepless night.

Naturally occurring chemicals called nucleotides that have previously been linked to sleepiness only reach their highest concentrations in human breast milk that is expressed at night.

Nucleotides are the building blocks of DNA, but they also participate in cellular signalling and metabolic processes within cells. Several of them have also been implicated in sleep.

The chemicals had already been found in breast milk, their concentrations increasing in the first few weeks after birth, so it seemed likely that they are important for tissue development. Now it seems they may have an additional role.


Milk round

Cristina Sánchez at the University of Extremadura in Badajoz, Spain, and her colleagues looked at the concentrations of 5’UMP, 5’AMP and 5”GMP – the three nucleotides most strongly associated with sleep and sedation – in the breast milk of 30 healthy mothers who had been breast-feeding for at least 3 months. Samples of milk were collected before each feed over a 24-hour period, with between six and eight samples collected per mother.

They found that concentrations of 5’AMP were highest at the beginning of the night, while levels of 5’GMP and 5’UMP increased as the night wore on. These sedatives were found at much lower concentrations in milk expressed during the day.

“It is a mistake for the mother to express the milk at a certain time and then store it and feed it to the baby at a different time,” says Sánchez.

She suggests that 5’AMP in breast milk might be fuelling the release of the sleep-promoting neurotransmitter GABA, while 5’GMP is involved in the secretion of melatonin,which helps regulate the natural body clock. 5’UMP is known to promote the amount of both REM and non-REM sleep.

Since previous studies have also hinted that bottle-fed infants have more problems sleeping through the night, Sánchez’s colleague, Javier Cubero, has been investigating whether sleep-inducing nucleotides could be added to formula milk.

In a separate study, Sánchez and Cubero created a “night-time” milk by adding 5’AMP and 5’UMP to standard formula milk. Infants receiving this milk between 6 pm and 6 am, and normal milk during the day, fell asleep faster and spent longer sleeping than when they drank standard formula milk all the time.

Journal references: Nutritional Neuroscience, DOI: 10.1179/147683008X344174; Neuroendocrinology Letters, vol 28, p 360