Light is a major regulator of circadian rhythm, especially in children, whose eyes are more photosensitive. Previous research demonstrates that exposure to bright light suppresses melatonin (the sleepiness hormone) production twice as strongly in children as in adults. Authors of a new report found that children exposed to a wide range of light intensities experienced melatonin suppression that may decrease sleep quality.

Light activates photosensitive cells in the retina that contain a pigment called melanopsin, which is most sensitive to blue light around 480 nanometers in wavelength. Melanopsin signaling is transmitted to a brain region called the suprachiasmatic nucleus, the master regulator of the circadian clock and a modulator of melatonin production. Previous research has demonstrated greater melatonin suppression in children exposed to home light levels (about 140 lux) compared to dim light (about 30 lux); however, additional studies are needed to understand how a wider range of light intensities affect melatonin suppression in young children.

The researchers recruited parents of 36 children with good sleeping habits and asked them to complete a sleep diary for their children for seven days to assess baseline sleep quality. On the eighth day, the investigators transformed the participants' home into a dim-light environment by covering windows and installing low wattage light bulbs. Children entered the dim environment 4.5 hours before bedtime and the investigators measured their salivary melatonin levels every 20 to 30 minutes until one hour past bedtime. Children woke up in the morning and spent the entirety of the ninth day in the dim-light environment. One hour prior to bedtime, the investigators exposed children to light from an LED panel while playing and sampled melatonin levels again. The investigators set the LED panel to a single light intensity between 5 and 5,000 lux for each child based on a randomized pattern.

During the one hour of light exposure prior to bedtime, salivary melatonin levels were suppressed by 69 to 98 percent across all light intensities; however, there was no overall dose-response. The investigators found that instead of light intensity and melatonin suppression changing at the same rate (as they do in a dose-response relationship), all light intensities induced high levels of melatonin suppression. Light intensities below 40 lux (i.e., dim light) tended to produce less melatonin suppression compared to intensities above 40 lux, but no other statistical trends were observed.

These findings show that young children are highly susceptible to light before bedtime, especially light brighter than 40 lux (roughly equivalent to the amount of light provided by a 30 watt incandescent light bulb in a typical child’s bedroom). Further research is needed to understand how melatonin suppression relates to sleep quality.

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• Watch sleep expert, Dr. Matthew Walker, discuss the importance of sleep for children and older adults in this clip from the FoundMyFitness podcast.