Preschoolers are highly susceptible to the physiological impacts of light at night


Even slight exposure to light can prompt the critical sleep-promoting hormone melatonin to plummet in preschoolers in the hour before bedtime, potentially disrupting slumber long after the light goes out, according to new CU Boulder research.

The study, published this month, is the latest in a series, funded by the National Institutes of Health, examining how the central body clock of young children is unique. It suggests that preschoolers are highly susceptible to the physiological impacts of light at night, and some children may be even more sensitive than others.

“Our previous work showed that one fairly high intensity of bright light before bedtime dampens melatonin levels by about 90% in young children,” said first author Lauren Hartstein, a postdoctoral fellow in the Sleep and Development Lab at CU Boulder. “With this study, we were very surprised to find high melatonin suppression across all intensities of light, even dim ones.”

Light: The body’s strongest time cue

Light is the body’s primary time cue, influencing circadian rhythms that regulate everything from when we feel tired or hungry to what our body temperature is throughout the day.

When light hits the retina, a signal transmits to a part of the brain called the suprachiasmatic nucleus, which coordinates rhythms throughout the body, including nightly production of melatonin. If this exposure happens in the evening as melatonin is naturally increasing, it can slow or halt it, delaying the body’s ability to transition into biological nighttime.

Because children’s eyes have larger pupils and more transparent lenses than adults, light streams into them more freely. (One recent study showed that the transmission of blue light through a 9-year-old’s eye is 1.2-times higher than that of an adult.)

“Kids are not just little adults,” said senior author Monique LeBourgeois, an associate professor of Integrative Physiology and one of the few researchers in the world to study the circadian biology of young children. “This heightened sensitivity to light may make them even more susceptible to dysregulation of sleep and the circadian system.”

Research in a ‘cave’

To quantify how susceptible they are, the researchers collaborated with Colorado School of Mines mathematician Cecilia Diniz Behn for a new study.

They enlisted 36 healthy children, ages 3 to 5 years, for a nine-day protocol in which they wore a wrist monitor that tracked their sleep and light exposure. For seven days, parents kept the children on a stable sleep schedule to normalize their body clocks and settle them into a pattern in which their melatonin levels rose at about the same time each evening.

On the eighth day, researchers transformed the children’s home into what they playfully described as a “cave”—with black plastic on the windows and lights dimmed—and took saliva samples every half hour starting in the early afternoon until after bedtime. This enabled the scientists to get a baseline of when the children’s biological night naturally began and what their melatonin levels were.

On the last day of the study, the young study subjects were asked to play games on a light table in the hour before bedtime, a posture similar to a person looking at a glowing phone or tablet. Light intensity varied between individual children, ranging from 5 lux to 5,000 lux. (One lux is defined as the light from a candle 1 meter, or about 3 feet, away).

When compared to the previous night with minimal light, melatonin was suppressed anywhere from 70% to 99% after light exposure. Surprisingly, the researchers found little to no relationship between how bright the light was and how much the key sleep hormone fell. In adults, this intensity-dependent response has been well documented.

Even in response to light measured at 5 to 40 lux, which is much dimmer than typical room light, melatonin fell an average of 78%. And even 50 minutes after the light extinguished, melatonin did not rebound in most children tested.

“Together, our findings indicate that in preschool-aged children, exposure to light before bedtime, even at low intensities, results in robust and sustained melatonin suppression,” said Hartstein.

What parents can do

This does not necessarily mean that parents must throw away the nightlight and keep children in absolute darkness before bedtime. But at a time when half of children use screen media before bed, the research serves as a reminder to all parents to shut off the gadgets and keep light to a minimum to foster good sleep habits in their kids. Notably, a tablet at full brightness held 1 foot from the eyes in a dark room measures as much as 100 lux.

For those children who already have sleep problems?

“They may be more sensitive to light than other children,” said LeBourgeois, noting that genes—along with daytime light exposure—can influence light sensitivity. “In that case, it’s even more important for parents to pay attention to their child’s evening light exposure.”

Neuroanatomy and physiology underlying biological and behavioral responses to light

The physiological effects of light are mediated by the eye in humans. Light entering the eye stimulates retinal photoreceptors that convert photic information into neuronal signals, which get transmitted via ganglion cells to various regions of the brain (Fig. 1). For many years, it was thought that there were only two classes of photoreceptors in the human eye—the rods and cones; however, another very different photoreceptor type was discovered in the mammalian eye about two decades ago.

These retinal photoreceptors are specialized ganglion cells that contain the photopigment melanopsin and are intrinsically sensitive to light, and were therefore called intrinsically photosensitive retinal ganglion cells (ipRGCs) (Berson et al. 2002; Hattar et al. 2002; Provencio et al. 1998, 2000).

Fig. 1. Schematic illustration of the neuroanatomical underpinnings of physiological effects of light. The intrinsically photosensitive retinal ganglion cells (ipRGCs) transmit environmental light information via the retinohypothalamic tract (RHT) to the central clock in the brain (SCN, suprachiasmatic nuclei); other direct projections of ipRGCs include thalamic and other brain regions. The response will depend on the light characteristics and/or other mediating factors. LGN: lateral geniculate nucleus; IGL: intergeniculate leaflet


From the retina, light information is transmitted to multiple targets in the human brain via two major pathways. The visual pathway employs the optic nerve, chiasm and tract, which sends information to structures involved in image formation, including the lateral geniculate nucleus (LGN), intergeniculate leaflet (IGL) and visual cortex of the occipital lobe.

The retinohypothalamic tract (RHT) is responsible for carrying light information from the retina to the suprachiasmatic nuclei (SCN) in the hypothalamus. The SCN serves as the biological clock in mammals and has numerous downstream connections with other central nervous system structures, including the spinal cord and brain (e.g. septum, thalamus, midbrain and other regions of the hypothalamus).

The RHT also projects to other nonvisual nuclei and regulatory centers of the brain that are independent of the circadian pacemaker (Gooley et al. 2003; Hattar et al. 2006).

Physiological effects of light: description and basic methods
Light can markedly influence a host of physiological functions, including circadian entrainment and phase shifting of the circadian system, neuroendocrine regulation, sleep, alertness, learning and memory, mood, and pupillary responses. In this manuscript, we mainly focus on studies of phase shifting, melatonin suppression, and alertness, as these functions have been the most extensively studied in both animal models and humans. It is important to keep in mind, however, that outside of the laboratory, each physiological effect of light does not occur in isolation; the same light may simultaneously influence all or some of these various functions. Thus, integrative lighting strategies must consider the broader effects of light on human health and performance. Multiple-outcome studies are limited to date, but will be useful to further guide future applications.

Phase shifting and entrainment

In humans, nearly every cell of the body contains molecular level rhythms, generated by cellular circadian clock machinery, which regulate cell metabolism, immune responses, DNA repair, and mitochondrial function (Sulli et al. 2018). Recent work has shown that beyond this clock in the cell, the SCN along with a network of clocks in peripheral organs, coordinate various physiological functions that result in circadian (“circa” meaning about and “dies” meaning a day) peaks and troughs in physiology, including core body temperature (CBT), hormone levels (e.g. melatonin, cortisol), energy metabolism patterns, reproductive cycles, and immune function variability across the day (see for example Pilorz et al. 2018, for an in-depth review). On a behavioral level, feeding-fasting, sleep-wake and rest-activity cycles, as well as fluctuations in cognitive function, are modulated by the circadian clock. Under ideal conditions, cellular, physiological and behavioral events are integrated and coherent at every level (Vetter 2020).

Light is the primary regulator of human circadian rhythms and a robust timekeeping signal, which together with a functional circadian system, will lead to stable entrainment (Roenneberg et al. 2013). Under normal conditions, circadian entrainment occurs via daily light-induced shifts that adjust for the difference between the endogenous period of the rhythm and the more precise 24-hour period of the solar light-dark cycle. Even just a single brief pulse of light is enough to shift the clock, and both the magnitude and direction of that phase shift is dependent on the timing of photic administration (Fig. 3). For example, light of sufficient potency early in the biological night elicits a delay in rhythms whereas light much later in the night will cause an advancing shift to an earlier time. Such shifts reflect the differential effects of light on the SCN at different phases of the clock in mammals, which are often described via phase response curves (PRCs) (Boivin et al. 1996; Czeisler et al. 1989; Honma and Honma 1988; Khalsa et al. 2003; Klein et al. 1991; Minors et al. 1991; Moore 1995; Pittendrigh et al. 1984).

Fig. 3. Phase Response Curve (PRC) for Light (Type 1). This schematic PRC illustrates that the magnitude of phase shifting by a light pulse will depend on the time of administration, and more specifically, on an individual’s biological (or circadian) time, which is typically based on the timing of the melatonin rhythm. Biological night refers to the time when melatonin levels are high. Figure credit: UC San Diego BioClock Studio, modified from Rueger et al., 2013.

Fig. 4.  Two spectral power distributions (SPDs) at 3500 K that are a metameric match to the CIE 1931 2° Standard Observer. Melanopic content refers to the short-wavelength proportion of the light source and its relative efficacy to stimulate melanopsin, the light-sensitive biochemical in the retinal ganglion cells

Neuroendocrine regulation

In addition to light’s capacity to shift the circadian phase of various human hormones, light exposure during the biological night can also acutely reduce the high nocturnal levels of circulating melatonin, which is produced and secreted by the pineal gland. While there are substantial inter-individual differences in absolute melatonin levels, nocturnal melatonin is robustly suppressed by bright light in most individuals (Arendt 2006; Bojkowski et al. 1987; Lewy et al. 1980; McIntyre et al. 1989).

Laboratory studies of melatonin suppression by light in humans involve collection of blood, saliva or urine at night, when the hormone typically rises in healthy individuals. Human studies employing the acute melatonin suppression response as a primary dependent variable are generally less labor- and time-intensive than the protocols required for assessment of circadian phase shifting, allowing for more powerful within-subjects experimental designs and increased replication within and between labs.


Light also has acute alerting properties similar to that observed after caffeine consumption (Wright et al. 1997). Specifically, light has been shown to significantly reduce reaction time and attentional lapses, decrease subjective sleepiness, improve alertness, and enhance performance on some neurocognitive tests (Badia et al. 1991; Cajochen 2007; Cajochen et al. 2011; Rahman et al. 2014; Souman et al. 2018).

Laboratory studies suggest that the alerting effects of light may vary by time of day, though this response has largely been characterized during the night (Rahman et al. 2014), and improvements in alertness may be minimal in well-rested individuals during daytime (Lok et al. 2018). It is also important to note that not all studies have identified a consistent enhancement of all measures of alertness and neurocognitive responses (Rahman et al. 2017; Segal et al. 2016; Sletten et al. 2017; Souman et al. 2018). Alertness is most commonly characterized via validated subjective alertness measures (e.g. Karolinska Sleepiness Scale, KSS, Akerstedt and Gillberg 1990) and the objective assessment of sustained attention (psychomotor vigilance test, PVT, Dorrian et al. 2004).

Such measures have the advantage of being sensitive to sleep deprivation with a large literature for comparison, though the generalizability to more practical outcomes is debated. Alerting and cognitive effects of light are often of special interest in the context of educational institutions and workplaces seeking to improve performance. Around-the-clock organizations, in particular, may aim to enhance nighttime alertness levels of employees, and thereby reduce error rates and increase safety. Few studies, however, have tested the alerting or performance benefits of intentionally designed architectural lighting solutions in relation to operational outcomes.

reference link :

More information: Lauren E. Hartstein et al, High sensitivity of melatonin suppression response to evening light in preschool‐aged children, Journal of Pineal Research (2022). DOI: 10.1111/jpi.12780


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