A Victorian researcher says the time that expressed milk is collected could affect babies' sleep patterns after feeding, due to the hormones present at the time their mothers pumped.
However, the Australian Breastfeeding Association says the preliminary study does not deliver enough evidence to suggest that mothers need to change their expressing practices.
La Trobe University Bendigo researcher, Lauren Booker, says milk expressed during the day has higher levels of cortisol, which is likely to make a baby more alert, while milk expressed at night has higher levels of melatonin, which helps initiate sleep.
Dr Booker has conducted a preliminary exploration into the impact of 'mistimed' expressed breastmilk feeding on infant sleep outcomes, involving 329 mothers with babies who were less than a year old, via an anonymous online survey.
"The theory is that there's this circadian timing in breastmilk which could potentially be playing a very important role in supporting the development of an infant's circadian rhythm," Dr Booker said.
"We found that infants that consumed the mistimed breastmilk took significantly longer — nearly 13 minutes longer — to get to sleep at night."
Researcher says babies could be 'jet-lagged'
Dr Booker says almost 70 per cent of mothers will express or pump breastmilk at some stage in their baby's life.
"A lot of mothers will express breastmilk at a certain time of day and then give that breastmilk at a time of day different to when they pumped it," she says.
"The infants could be feeling jet-lagged because they're getting melatonin during the day … making them really drowsy, and then be getting cortisol at night and make them wide awake.
"There's a potential for misalignment of the hormones, potentially causing sleep problems."
Dr Booker says a baby's circadian rhythm is developed in the first three to six months.
"About 21 per cent of the children who have sleep problems at infancy will still have sleep problems in adolescence and adulthood," she says.
However, she says there's a workaround for 'mistimed' breastmilk – saving day milk for day feeds, and night milk for night feeds.
"Simply just labelling the time of day that you pump and make sure you give it to the infant at the same time of day that you pumped – you just knock out that problem altogether," Dr Booker said.
More research needed, association says
Senior manager of breastfeeding information and research at the Australian Breastfeeding Association Naomi Hull welcomed the new research into breastfeeding and breastmilk.
"But because this is a preliminary study, and the results don't show anything significant at this time, we are not suggesting that mothers need to change any of their practices around expressing breastmilk at this time," she said.
"Expressing to provide breastmilk for your baby is already quite a lot of work.
"We would not want to impose any additional tasks on mothers that are already doing this incredibly important work for their baby, particularly when variations in hormones may be present, but the impact of that is probably quite minimal in comparison to the importance of breastfeeding and breastmilk for mothers and babies."
The association would like to see more robust research into the theory.
"This is a very preliminary study, and only based on a small sample," Ms Hull said.
"The researchers themselves have highlighted areas that would need to be looked at further."
Researcher says more funding needed
Dr Booker says the research she's completing is novel. She is not aware of anyone else in Australia working on a similar project.
"It's been quite challenging to get any funding," she said.
She studied circadian rhythm misalignment disorders in shift workers for her doctorate and developed her theory about breastfeeding after having her daughter.
"I realised that the composition of breastmilk changed throughout the day and night," Dr Booker said.
"I then thought about, 'Well, how about shift workers and mothers that go back to work, and they then have a disruption in their circadian timing because of shift work – would that then alter the circadian timing of the breastmilk, and then affect the infant's sleep?"
She hopes to secure further funding to objectively measure the outcomes.
"Proof of concept does show there's something there – there is an issue – and it would be great to measure that further," Dr Booker said.
