We live in a 24-hour environment, in which light and darkness follow a diurnal pattern. Our circadian pacemaker, the suprachiasmatic nuclei (SCN) in the hypothalamus, is entrained to the 24-hour solar day via a pathway from the retina and synchronizes our internal biological rhythms. Once we come to this world, one of the first things we need, is food. Naturally, as mammals(from Latin mamma “breast”), we feed ourselves from the breast. But, Human breast milk is more than a meal — it’s also a clock, providing time-of-day information to infants. The composition of breast milk changes across the day, giving energizing morning milk a different cocktail of ingredients than soothing evening milk. Researchers believe this “chrononutrition” may help program infants’ emerging circadian biology, the internal timekeeper that allows babies to distinguish day from night.
What happens, though, when babies drink milk that does not come directly from the breast but is pumped at different times of the day and stored in advance of feeding? Scientists have rarely considered the potential effects of “mistimed” milk on infants’ development, but the implications are potentially far-reaching.
In the same way, rhythmic variations in ambient illumination impact behaviors such as rest during sleep and activity during wakefulness as well as their underlying biological processes. The availability of artificial light has substantially changed the light environment, especially during the evening and night hours. Phones, laptops, ipads, and more around the babies. This may increase the risk of developing circadian rhythm sleep-wake disorders (CRSWD), which are often caused by a misalignment of endogenous circadian rhythms and external light-dark cycles. On the other hand, light can also be used as an effective and non-invasive therapeutic option with little to no side effects, to improve sleep, mood, and general well-being.
The architecture of the circadian system
The central master-clock in mammalian species is a paired structure in the hypothalamus with a volume of just about 0.25 mm3 per nucleus. Within the mammalian SCN, a molecular oscillator keeps the clock oscillating at its normal pace. The basis of this oscillator is two interconnected molecular feedback loops of clock gene expression, a detailed description of which is beyond the scope of this review though.
Successful interaction between body and environment however needs more than just a central clock; it also requires input pathways relaying information about the environment and the body to the SCN to achieve adequate entrainment as well as output pathways communicating timing information to the body to synchronize bodily processes with the circadian phase
Sleep, eating, and energy levels all show circadian rhythms, which means they follow a daily cycle. As any parent who has sleepwalked through a 3 a.m. feeding knows, infants are not born with these rhythms fully set. Instead, their sense of day and night develops over the first weeks and months of life, thanks to cues like sunlight and darkness.
Babies vary: Some show predictable circadian fluctuations in hormones linked with alertness, sleep, and appetite, and can sleep for long stretches shortly after birth, whereas others seem to have their daily rhythms upside-down for months. Delays in the development of circadian biology can increase the risk of colic and lead to growth and feeding problems.
Breast milk may help program infant circadian rhythms, helping to explain why some parents of newborns enjoy long full nights of sleep, whereas others struggle to get their infants on a schedule.
Breast milk changes dramatically over the course of the day. For example, levels of cortisol — a hormone that promotes alertness — are three times higher in morning milk than in evening milk. Melatonin, which promotes sleep and digestion, can barely be detected in daytime milk, but rises in the evening and peaks around midnight.
Night milk also contains higher levels of certain DNA building blocks which help promote healthy sleep. Day milk, by contrast, has more activity-promoting amino acids than night milk. Iron in milk peaks at around noon; vitamin E peaks in the evening. Minerals like magnesium, zinc, potassium, and sodium are all highest in the morning.
Daytime milk may pack a special immune punch. Among mothers who provided researchers with milk samples across the first month postpartum, immune components — including key antibodies and white blood cells — looked higher in day milk compared to night milk. Another study found higher levels of a component important for immune system communication in day milk compared to night milk.
While it’s clear that milk changes over the course of the day, scientists know little about what this means for infant health. Researchers do know that the hormones and immune components in breast milk are passed along to infants and that infants are starting to develop and refine their own circadian rhythms during the first months of life. It’s plausible that the chronosignals in breast milk would help to shape infants’ own circadian biology. Differences in infant feeding patterns might help explain why there’s such variability in the development of these daily rhythms from one infant to another.
Fundamentals of light
To understand the effects of light on human physiology, it is important to understand light. Briefly, light is radiation in a specific range of the electromagnetic spectrum.
The spectrum of daylight, which is light from the sun filtered by the atmosphere is relatively broadband in its distribution. The availability of daylight depends on geographical location and season. In the timeframe of human evolution, it is a rather recent development that light can be available during all times of day through artificial light. Artificial light allows for illuminating indoor and outdoor spaces. It comes in many forms, e.g. incandescent, fluorescent, or light-emitting diode (LED) lighting.
While light generated by these technologies may all appear “white”, the underlying spectra are rather different.
The reason why many different types of spectra might have the same appearance lies in the retina. Critically, different spectra, even if they create the same visual impression, may vary in their chronobiological effects on the circadian clock.
Recently, the Commission International de l’Eclairage (CIE), the international standard body for quantities related to light, issued a new standard containing a reference framework for quantifying the effects of light on non-visual functions.
Effects of LED light on the circadian clock
Two effects of light have been interrogated extensively in human circadian and sleep research: (1) the acute suppression of melatonin in response to light exposure and (2) the ability of light exposure to shift circadian phase.
The system mediating melatonin suppression has a spectral sensitivity that is broadly consistent with the spectral sensitivity of melanopsin. Similarly, the spectral sensitivity of circadian phase-shifting shows its maximal effect near the peak spectral sensitivity of melanopsin.
The effects of light on the phase of the circadian clock depend on the timing of light exposure. This is formally summarised in the phase response curve (PRC), which describes the amount of phase shift (in minutes and hours) achieved by exposure of light at a given circadian phase. Roughly speaking, the effect of morning light is that it advances the clock, while evening and night light delays the clock.
Both melatonin suppression and circadian phase shifts are modulated by the “photic history”, i.e. the amount of light seen during the day. The long-term adaptive influences of the “spectral diet” in the real world remain an important area of investigation.
Effects of light on sleep
The human sleep-wake cycle, which is periods of sleep during the night and wakefulness during the day, is one of the most prominent examples of a circadian behavioral pattern, especially for babies. It results from the interaction between two factors: the circadian drive for wakefulness and the homeostatic sleep pressure. The activity of the circadian pacemaker is aligned to counteract the increasing sleep pressure resulting from sustained wakefulness during the daytime. Likewise, the nocturnal increase in circadian sleep tendency counteracts the decrease in sleep propensity resulting from accumulated sleep thereby supporting a consolidated phase of nocturnal sleep.
Breast milk, artificial lighting, smartphones, and visual display units
In addition to natural daylight, babies are nowadays also exposed to a considerable amount of artificial light. This is particularly the case in the evening hours, i.e. when the circadian system is most sensitive to light-induced phase delays. Thereby, light therapy is more efficient to delay the timing of the circadian clock and thus sleep.
Even thou, mothers can label their milk with the time it was pumped and coordinate infant feedings to offer morning milk in the morning, afternoon milk in the afternoon, and night milk at night, they keep the constant use of visual units around the baby. The use for the babies is not different, entertainment as well
So, which one is better?
Is always about finding the balance. Rather than only use one of the methods, the responsible practice of light therapy and adequate alimentation of your newborn should be combined to get on track the circadian system