Is that dream feed messing up your baby’s body clock?
Back in 1957, research suggested 70% of babies began sleeping through the night at age three months. You have probably been told this by past generations of mothers, health professionals and others who have passed this along as fact for more than sixty years.
But here is what they leave out:
- Sleeping through the night was defined as not disturbing the parents between 12am and 5am!
- Although 30% of babies were not “sleeping through” at 3 months, the information was misinterpreted to suggest all babies were capable of - and therefore should be – doing so.
- Although those babies were reported to be “sleeping through” at 3 months, half of them reverted back to waking again!
- At the time of the study, most babies were formula-fed, supplemented with rice cereal and other solids much earlier and slept in a separate room to their parents.
This study - which is still frequently referenced close to seventy years later was - at best - a snapshot in time of a group of babies deemed typical at the time through the lens of scientists.
The question of how to define the night was not easy to decide. In the end we confined our attention to cases where the baby woke and cried or fussed between midnight and 5 a.m. at least once a week.
Ask any parent what their goals are for infant sleep and I bet they expect more than that five hour window once a week! Indeed, many sleep trainers promise an uninterrupted 7pm to 7am - seven hours more than this study concluded a definitive time period!
Infants are no different to the rest of us: their bodies are run by hormones around the clock. We all have a circadian rhythm which develops around three months of age and this is what wakes us up and sends us to sleep. Two hormones work as a tag-team, switching on and off in reponse to daylight or lack of it:
Cortisol is well-known for its role as our “stress hormone”. But the release of cortisol as the sun rises to motivate our brains to get out of a cosy bed and work all day. Our inclination to push the snooze button and sleep in is our concious brain protesting this natural alarm system. One reason we surround ourselves at night with curtains and blinds to block out the morning sun is an effort to trick our brain into thinking it is still the middle of the night. But if we allow ourselves to sleep with natural light coming through the windows at dawn, our brains will release the cortisol to start the day. If we try to sleep in the daytime due to shift work or a late night, we need to darken our sleep space to fool the brain. And more people are well-aware now of the impact of screens emitting blue light on sleep patterns.
Melatonin is often thought of these days as a sleep-inducing medication for people struggling with insomnia. It is, however, a hormone which we all produce in response to less daylight towards the end of the day. Our brain registers darkness as a signal to prepare for sleep. With less cortisol release and the growing effect of melatonin, we begin to get sleepy in the evening and are ready for sleep by night. Artificial lighting can mess with this process, with blue light from screens and other sources tricking our brain into thinking it is still daylight and time to work.
When we look at this research from 1957, we must do so with the knowledge that the people studied were very unlikely to be breastfed as formula feeding increasing in use around this time. Alongside this, babies just a few weeks old were being given canned purees and cereals as the baby food industry recommended. It was even common to mix cereal or broken up Arrowroot biscuits into formula in bottles, in the hope babies would sleep longer. Which they might do, as their digestive systems struggled with foods they were not designed to process. In particular, the type of protein in these cow-milk based formulas was especially tough to breakdown, unlike the proteins in human-milk. The sedating effect necessary for herd animals like cattle has a similar impact on human babies, sending them into unnaturally long and deep sleep periods.
By contrast, human milk adapts around the clock to support the development of natural circadian rhythms. Breastmilk has its own pattern of cortisol/melatonin cycles. Research has shown that not only are melatonin levels in breastmilk higher at night, they are 5 times higher than daytime levels. And further studies have shown melatonin levels peak around 3am, with 10 times the levels seen in the afternoon! And the other side of the day? Morning levels of cortisol are four times higher than milk produced in the evening.
Babies who are formula fed lack the natural support in breastmilk to regulate their sleep patterns around the clock. Even that dream feed at bedtime of breastmilk expressed in the morning (part of some sleep trainer’s “strategies” to promote infant sleep could backfire: you might be feeding your baby a shot of cortisol just when they need higher melatonin levels! ) just might be keeping your baby awake!
"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.”
Bendigo researcher says 'mistimed' bottle feeds could affect baby sleep cycles
Now. let’s go back to that study which looked at babies sleep between 12 am and 5am:
The typical sleep pattern for breastfed babies can include cluster feeding in the late afternoon/early evening - as the sun wanes and moves towards setting, depending on the season. Before the wide-spread use of artificial lighting, the main meal would generally be eaten earlier in the day, allowing cooking and food preparation to be done in daylight. A lighter, simple meal would be served in the afternoon, with a final simple supper before the family headed to bed. This pattern of family meals would work well alongside the higher needs of the breastfed baby. A longer period of sleep between feeds often follows this time, allowing the mother to sleep before the onset of night feeds. Maternal - and therefore breastmilk - levels of melatonin peak in the early hours, and then gradually decrease until early morning.
For breastfed babies, melatonin levels peaked a few hours after their mothers’ tryptophan levels did. For formula-fed babies, the peak in melatonin happened much later, and infants spent significantly less time sleeping at night. source
Interestingly, maternal levels of prolactin, the hormone which stimulates milk production in response to milk removal, has its own cycle around the clock - which is very similar to that of the circadian rhthym. Prolactin levels peak around the 2am - 5am period when melatonin levels peak - and reach the lowest levels in the late afternoon/early evening … just as melatonin levels begin to increase. Cluster feeding is too often mistaken as a problem (which it is at a social level!) when it is really part of a system which sets up mother and baby for the coming night.
Frequent feeding late in the day stimulates prolactin production in the mother and transfers the increasing melatonin levels in her bloodstream into her breastmilk, signaling to the baby’s brain night is coming. In the morning, fueled by those heightened prolactin levels, breastmilk production is at its peak, and melatonin in the bloodstream is decreasing as cortisol increases and mother and baby wake up for the day.
Avoid those block out blinds during the day!
The sleep training industry and self-styled “baby whisperers” are big fans of using block out blinds during day time naps for babies and toddlers. There is no evidence these have a positive influence on sleep and might actually damage the day/night pattern.
During a total solar exclipse, birds and animals become confused and anxious when daylight disappears in the middle of the day. It messes with their body clock and they become unsure what to do. Humans have always spent much of the daylight hours outdoors and exposed to natural light from dawn. Hunter-gatherers and subsistence farmers would make the most of longer days in summer and then almost hibernate in winter. It was the Industrial Revolution which took adults and children off the streets and land, corralling them indoors within factories, mills, mines and schools. Before this, even the very rich knew the value of spending part of each day outside, creating pleasure gardens to make the experience more enjoyable. Babies in Victorian times were regularly walked outside in the new perambulators and it is still common in Scandinavian countries for babies to nap outdoors, well-rugged up against the cold. It is only in the past couple of decades that creating a completely dark sleep space has been popularised. Another modern idea is draping muslin wraps over prams when babies are sleeping out and about. Again, there is no evidence this is supportive of natural sleep and reduces daylight reaching the infant brain.
Consider your own circadian rhthym
It is clear the maternal cortisol/melatonin cycle influence babies in the womb during the last trimester. We can see that breastmilk has a role in regulating the infant sleep pattern, with its hormone levels matching those in the mother’s bloodstream. So, if a mother is artificially altering her own circadian rhythm through exposure to blue light later in the day and not getting early exposure to daylight in the morning - how does that impact on the infant’s day/night cycle? The answer is - we don’t know yet. The rapid increase in blue light exposure since the advent of personal devices which rarely leave our hands is now recognised but most of the attention focuses on teens, children and young adults. However, the infants in our arms are exposed to the same light as the adults caring for them: bright household lighting, large-screen TVs, laptop computers, tablets and phones. Evening lifestyle choices and behaviours impact the maternal melatonin cycle, which in turn impacts that of their baby. Sharing that bright light in the evening may also delay the release of melatonin in the baby or toddler. Look into blue light filters for your screens, create an end of day routine of red-lighting in all rooms and - hardest of all - limit screen use in the hours before sleep. Consider listening to podcasts and audio books instead, still enrtainment but without the wake-up call to your brain.
