YES – « In full light, not only would children find it much harder to fall asleep, but they would also wake up at the slightest farting fly ».

NO – « A nap in the dark risks scrambling children’s brains and disrupting their ability to fall asleep at night at home (enough to have, on its back, the parents’ union at the end of its rope)! »

Answer: BOF BOF. To help children fall asleep at night, it’s essential that they are exposed to daylight throughout the day, especially during the post-meridian break. During a nap in the dark, the child’s brain secretes melatonin – even though it’s not the right time – which can mess up the toddler’s circadian rhythm.

Why is it better for children to sleep in daylight rather than in the dark?

Daylight plays a key role in the timing of their biological rhythms. Natural light is what chronobiologists call a time-giver, a powerful signal that tells our bodies when it’s day or night, morning or evening. Daylight is a major synchronizer of our circadian rhythms[1]. In other words, daylight may or may not trigger the secretion of a particular hormone, for example. It’s no laughing matter!

In the dark, the brain – which isn’t very smart – thinks it’s night and secretes melatonin. During the day, daylight naturally inhibits the secretion of melatonin, the hormone of darkness par excellence[2]. But when a child’s body is plunged into darkness for an entire nap, the brain thinks it’s night. Melatonin is then secreted, and signals are scrambled: « I don’t get it anymore, guys, is it night or day?! Who closed the curtains? » grumbles the epiphysis responsible for secreting melatonin.

Children’s biological rhythms are in danger of being turned upside down (as is that of the adult on nap watch, for that matter!). Melatonin, which should begin to be synthesized at dusk, is likely to be secreted at a later time, making it more difficult for children to fall asleep in the evening, and adversely affecting the quality of their sleep at night. Beyond regulating their sleep, the correct synchronization of their internal clock plays a key role in their health, hormonal cycle and immune system. Basically, when the internal clock is disrupted, children’s entire metabolism is at risk of being deregulated!

What happens when a child is plunged into darkness at naptime?

First step: the child’s eye perceives a very, very, very low-intensity light signal (in the middle of the day, that’s weird!). The child’s retina, and more specifically the retinal ganglion cells[5], process and take charge of this light signal via a kind of high-performance eye camera called a « photoreceptor ».

Second step: this light signal is transmitted to the internal clock[6].

Third stage: after several relays, this light signal makes its way to its mate, the pineal gland, which is responsible for secreting melatonin, depending on the intensity of the light signal (some chronobiologists consider melatonin to be the hand of the clock!).

Fourth and last: the light signal is very weak, too weak, so melatonin – the hormone of darkness par excellence – is secreted… when it’s just past midday and the sun is at its zenith, halfway between dawn and dusk!

What’s the circadian rhythm?

No connection with the circus (too bad!). A circadian rhythm is a 24-hour cycle that groups together the biological processes of our wonderful body: the sleep-wake cycle (the best-known), but also variations in our body temperature and cortisol levels. The latter are just as cyclical as the day-night alternation, i.e. their levels rise and fall in a predictable and regular way over the same 24-hour day. Melatonin, for example, begins to be secreted in the middle of the evening as light levels fall, and peaks between 3 and 4 a.m. Cortisol, on the other hand, reaches its peak every day, just before waking up, between 6 and 8 a.m. (it’s thanks to cortisol that your body finds the energy to get you up and running in the morning!)

Ultradian rhythms, on the other hand, have a period of less than 24 hours (e.g. the alternation of REM and slow wave sleep). Subdian rhythms, on the other hand, have a period of more than 24 hours (as in our wonderful and memorable menstrual cycle, ladies!).

Between you and me, lighting up at night isn’t exactly the idea of the century either…

Conversely, much research has focused on the harmful impact of light at night on our melatonin levels and, on a larger scale, on our metabolism and health. A 2013 study[3] found that one hour’s exposure to screen light did not significantly affect melatonin levels. However, beyond two hours, melatonin levels were reduced by around 22%, a level that corresponds to exposure to daylight. In this particular case, the circadian rhythm evolved from a state of somnolence (as expected at night) to an abnormal state of alertness (as is usually the case during the day). According to a 2014 study published in theAmerican Journal of Epidemiology, sleeping with a light on at night can lead to a risk of being overweight[4]. Other research has found an association between night lighting and an increased risk of depression and cancer. In fact, the International Agency for Research on Cancer (IARC) classifies night work (involving lighting and activity normally reserved for the daytime) as « probably carcinogenic ».

CONCLUSION During the siesta, you can simply draw the curtains (to avoid one of the little humans getting a sunbeam in the retina) without completely blocking out the daylight… !

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[1] Did you raise an eyebrow at the word « circadian »? Then go and read the sidebar « What is a circadian rhythm?

[2] To find out more about this hormone, see the box in this chapter dedicated to it!

[3] Wood, B. et al. (2013). Light level and duration of exposure determine the impact of self-luminous tablets on melatonin suppression. Applied Ergonomics, 44, 2, 237-240.

[4] Emily McFadden, Michael E. Jones, Minouk J. Schoemaker, Alan Ashworth, Anthony J. Swerdlow, The Relationship Between Obesity and Exposure to Light at Night: Cross-Sectional Analyses of Over 100,000 Women in the Breakthrough Generations Study, American Journal of Epidemiology, 180, 245-250.

[5] Don’t hesitate to bring out this smart-looking notion at your next family dinner to impress your mother-in-law 😉

[6] The internal clock is located in the suprachiasmatic nuclei of the anterior hypothalamus – basically, somewhere in our brain!