A Baby's Internal Clock: How a Mother's Exposure to Light and Sleep Patterns Might Influence Prenatal Development

Even before birth, a baby’s body begins preparing for the transition between day and night. Genes aren’t the only factor at play here: the mother’s internal clock, her sleep-wake rhythms, and hormonal signals may also play a role in the development of the baby’s circadian system. Chronobiology examines how this connection works and why the natural day-night rhythm may already be significant during pregnancy. Why do some newborns sleep a lot during the day and are more often awake at night? And why does it take a few weeks after birth for a discernible day-night rhythm to develop? Part of the explanation lies in the development of the internal clock—a biological system that regulates the timing of our sleep, hormone production, and numerous other bodily functions.

How a Baby’s Internal Clock Develops

What many people don’t know: The foundations of this system develop even before birth. During pregnancy, the unborn child receives various biological signals from the mother’s body. These include hormones, changes in body temperature, and other rhythms that vary throughout the day and night. Of particular interest is the interplay between light, sleep, and melatonin. This hormone is released in greater quantities in the dark and signals to the body that biological night has begun. Since maternal melatonin can cross the placenta, the unborn child also receives information about the time of day. Researchers therefore assume that the mother’s internal clock plays a role in the development of the child’s rhythms. However, the extent to which individual factors influence this process in humans and what long-term consequences deviations may have has not yet been conclusively clarified.

The human body’s central internal clock is located in the brain, more specifically in the suprachiasmatic nucleus of the hypothalamus. It coordinates numerous biological rhythms and is primarily guided by the light-dark cycle of the environment. In adults, light reaches this central control center via the eyes. There, information is processed that influences, among other things, the sleep-wake cycle and melatonin production. In the unborn child, this system is still developing. The biological clocks of individual cells and organs mature gradually. At the same time, the fetus receives signals from the mother’s body that can provide clues about the time of day.

This does not mean that a baby in the womb already has the same sleep-wake cycle as an adult. Its periods of activity and rest also follow shorter biological cycles, and the organization of sleep changes significantly during pregnancy. The development of the internal clock is therefore not a single point in time, but a process that begins before birth and continues to develop after birth.

Melatonin: How the Mother’s Body Signals the Time of Day

Melatonin is one of the most important hormones when it comes to biological night. Its secretion typically increases in darkness, while light can suppress its production. During pregnancy, melatonin can cross the placenta and reach the unborn child. This provides the fetus with a biological signal linked to the mother’s day-night rhythm. Researchers believe that this signal may help synchronize the developing fetal system. However, melatonin does not serve solely to regulate sleep. It is also being studied in connection with other biological processes, including the development and function of the placenta as well as antioxidant mechanisms.

Exactly how these various functions interact in humans during pregnancy is the subject of ongoing research. Much of the detailed knowledge about these mechanisms has so far come from animal models or experimental studies. These findings cannot be readily applied to every pregnancy. It is therefore crucial to distinguish between a plausible biological mechanism and a health effect that has already been proven. It is well established that maternal melatonin can cross the placenta. However, the significance of individual changes in the melatonin rhythm for the child’s later sleep development has not yet been fully clarified.

Why Evening Light Might Play a Role

Light is the most important external cue for the body’s internal clock. It is not only the brightness that matters, but also the timing of light exposure. Morning daylight helps the body recognize the biological day. Bright light late in the evening, on the other hand, can suppress melatonin production and delay the onset of the biological night. This is particularly relevant when the natural alternation between light days and dark nights is permanently altered. In modern living environments, many people spend a large part of the day indoors and are exposed to artificial light in the evening. Smartphones, computers, and bright room lighting can further alter the lighting environment.

During pregnancy, this raises the question of whether such lighting patterns not only affect the mother’s sleep but also alter the temporal signals she transmits to her unborn child. A scientific review article published in 2026 examined artificial light at night, melatonin, and potential effects on the health of mother and child. The research analyzed suggests that disrupted circadian rhythms may be associated with various adverse pregnancy and developmental outcomes.

However, much of the evidence regarding direct effects on the fetus comes from animal studies. Furthermore, observational studies in humans cannot always clearly distinguish between the effects of light, sleep deprivation, shift work, stress, and other influencing factors. It would therefore be too broad a conclusion to state that evening smartphone use demonstrably harms the unborn child. Rather, the interesting question is how the regularity of the mother’s day-night rhythm might support biological development.

What Happens if the Mother’s Sleep Rhythm is Irregular?

Pregnancy often alters sleep patterns. Nausea, back pain, frequent urination, hormonal changes, and physical discomfort can cause expectant mothers to wake up more often at night or sleep at different times than before. However, irregular sleep does not automatically mean that the unborn child’s internal clock is also disrupted. Although sleep duration and the circadian rhythm are related, they are not the same thing.

The internal clock is primarily regulated by temporal cues. These include light, hormones, and recurring routines. This is why people can have a shifted biological rhythm despite getting enough sleep—for example, when bedtime varies frequently or when working night shifts.

Shift work, in particular, has been the subject of research. Some epidemiological studies have found associations between night work or disrupted daily rhythms and adverse pregnancy outcomes, including preterm birth and lower birth weight. However, such observations do not prove that the circadian disruption alone is responsible. Workload, physical exertion, and other life circumstances may also play a role. For pregnant women who work shifts, this does not automatically mean they have to give up their jobs. Rather, it makes sense to discuss individual stressors and potential risks with their doctor.

Why Babies Don’t Yet Have a Fixed Day-Night Rhythm After Birth

Many parents expect their newborn to be able to distinguish between day and night after birth. In fact, the circadian system is not yet fully developed at this stage. Newborns sleep in several phases spread throughout the day. Hunger, feeding, physical development, and other needs initially influence their sleep rhythm more strongly than a stable 24-hour cycle.

It is only in the following weeks and months that differences between day and night become more pronounced. Both the maturation of the child’s body and the environment play a role in this process. After birth, the baby now receives light signals directly through its eyes. Daylight, nighttime darkness, and recurring routines help to gradually organize the sleep-wake rhythm.

Melatonin levels also change. While the unborn child receives signals from the mother, the baby’s own hormonal rhythms develop only gradually after birth. That is why it is normal for newborns to wake up frequently at first and not yet have reliable nighttime sleep. Restless sleep in the first weeks of life is therefore not automatically a sign that something went wrong during pregnancy.

How Expectant Mothers Can Support Their Baby’s Internal Clock

Research suggests that regular daily rhythms are beneficial for circadian health. However, there is currently no specifically validated method that pregnant women can use to specifically optimize the development of their baby’s internal clock. Nevertheless, certain everyday habits can help support their own sleep-wake rhythm:

• Get some daylight in the morning: Spending time outdoors can help set the internal clock for the start of the day.
• Create a calmer lighting environment in the evening: Dim lighting in the hours before bedtime supports the natural transition to nighttime rest.
• Stick to as regular a sleep schedule as possible: Consistent bedtimes and wake-up times can promote a more stable daily rhythm.
• Ensure adequate rest: If pregnancy-related symptoms are consistently disrupting your sleep, this should be discussed during your prenatal care visits.
• Discuss shift work on an individual basis: If you frequently work night shifts or rotating shifts, a medical consultation can help assess potential stressors and options for adjustment.

These recommendations are primarily intended to support maternal sleep and circadian health. They do not guarantee that the baby will sleep better later on or that certain pregnancy complications can be prevented. It is also important to note that melatonin supplements should not be taken on one’s own during pregnancy to improve sleep or influence the child’s development. The available data are insufficient to recommend routine use.

Conclusion: The Biological Clock Does Not Begin Only After Birth

The development of a child’s day-night rhythm begins even before birth. The unborn child’s organism matures in an environment where light, hormones, and numerous other biological signals change throughout the day. Maternal melatonin is a key potential mediator between the mother’s internal clock and the developing fetal system. Exposure to light and regular daily rhythms are also being studied as influencing factors.

However, it is not yet fully understood to what extent the mother’s individual habits actually influence the child’s later sleep development. Research provides evidence of biological connections but does not offer a basis for assigning blame when sleep is irregular during pregnancy.

For expectant mothers, this means above all that maintaining as regular a daily routine as possible, getting sufficient rest, and ensuring a pleasant lighting environment can support their own well-being. At the same time, the development of the baby’s internal clock remains a complex biological process that cannot be controlled by a single measure.