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How Does Light Affect Sleep: The Circadian Rythm

Updated: Jun 1, 2023

Quick, without looking it up, what’s the opposite of nocturnal?

We all know what nocturnal means, right? Animals that sleep during the day and do most of their activities at night – bats, owls, and raccoons are some familiar examples. It always surprises me that we’re not nearly as familiar with the term diurnal (that’s the answer, in case you didn’t know.)


As humans, that’s what we are, diurnal. We take care of business during the day and get our much-needed rest after the sun goes down. Interestingly, we’re in a significant minority in that regard. Around 80% of mammals evolved to sleep during the day.

Why we evolved this way is likely to remain a bit of a mystery. Although plenty of people claim to be “night owls,” biologically, we humans are built to do our modern versions of hunting and gathering in the daylight. Our eyes don’t adapt to the dark all that well, we don’t have the echolocation skills of the bat, and we rely on the sun vitamin D. And, since evolution is a painfully slow process, that’s the way it’s going to be for another couple million years, at least.

But some really sweet benefits come with being daytime creatures, and one of my favourites is a little thing called the circadian rhythm.


What is circadian rhythm?

The circadian rhythm, as you may already know, is the internal clock in the human body that prompts us to wake up in the morning and go to sleep at night, and much like an actual clock, it has a LOT of moving parts, only instead of gears and springs, it’s made up of stimuli and hormones.



Diagram of circadian rhythm

Two of these hormones are going to play the starring roles in our story, and those are melatonin and cortisol. If you’ve got a baby having a hard time sleeping, you have undoubtedly heard a lot about both of these guys.

What is melatonin?

Melatonin is produced in the pineal gland of the brain, and its role is to help the body relax, both mentally and physically, and help us get to sleep and stay asleep. So when your baby gets an 11-12 hour stretch of glorious sleep, you can thank their pineal gland for firing up those melatonin pumps.


Melatonin production

But you can also thank the daylight because exposure to the sun stimulates melatonin production. Production, mind you. Not secretion. We’ll talk about that in a second, but the buildup of the hormone itself is stimulated by exposure to sunlight. So it’s not just an old wives tale. Getting your baby outside during the day really does help them sleep better at night!



Pineal Gland diagram



Once nighttime rolls around, the sun goes down, and our eyes stop taking in light, the brain responds by releasing those stores of melatonin that it built up during the day. That signals our muscles to relax, tells the brain to ease back on the thinking, and allows us to drift peacefully off to sleep, hoping for a long, restful night.

Come morning, the blue light from the sun starts to permeate the thin skin of our closed eyelids, signalling the brain that it’s time to get back into gear. After all, we’ve got hunting and gathering to do!

So now our brain is going to help us get out of bed, shake off those cobwebs, and get on with our tasks for the day, and it’s going to do that, in part, by telling our adrenal glands to pump out some cortisol.


What does cortisol do?

Now, cortisol gets a bad rap, in my opinion, because people associate it with stress. This is especially true if you have a baby at home because crying, stress, and cortisol all get packaged together in a lot of everyday conversation. “Baby’s crying? That’s because their cortisol levels are elevated, causing them stress. Or maybe it’s the other way around. They’re stressed, making them cry, and spiking their cortisol levels. Some combination of stress, cortisol, and crying. That’s your baby’s issue.”

The truth is cortisol is a very beneficial hormone. It regulates metabolism, blood pressure, and blood sugar, suppresses inflammation, and regulates the body’s stress response. It’s not some toxic stimulant that causes us to freak out. It has a ton of benefits, one of which is that it perks us up and keeps us alert during the day.



Cortisol  Steroid Hormone Diagram

Blue light and sleep

This whole intricate dance between light and dark, cortisol and melatonin, awake and asleep, evolved over an incredibly long time, and it worked like magic up until, relatively speaking, very recently, when we discovered that we could pass an electric current along a filament and “artificially” illuminate our surroundings. Before that, we relied exclusively on fire, which emits very little blue light.

Depending on their hue, light bulbs emit quite a bit of it. And TVs, LEDs, computer monitors, iPads, smartphones, and other screens that surround us today flood our eyes with it. Unfortunately, all that blue light coming at us in hours when we would normally be enveloped in darkness signals the brain that it’s still daytime and inhibits the release of melatonin, making it harder to get to sleep.

Since we can’t reasonably get rid of all of the sources of blue light around us, the best thing to do for our little ones’ sleep is to turn off those intense sources, like TVs and smartphones, a couple of hours before they go to bed, and make sure their sleeping area is as dark as we can get it. I’m talking really dark! Can’t see your hand in front of your face, kind of dark. Some blackout blinds can be a game changer, especially if you live somewhere where the days get exceptionally long in the summer.

So that’s the breakdown of the circadian rhythm and its daily heroic effort to keep us running at peak performance. It really is a fascinating little piece of our physiology, and with just a little support from our side, can work wonders in getting us out of bed with energy and enthusiasm and helping us feel relaxed and peaceful when it’s time to sleep. Work with it instead of resisting it and I guarantee you’ll start seeing and feeling the results immediately.


This article was written by Richelle Franklin our sleep Expert from Sleep Right Tonight. Richelle has videos and expert articles as part of our Parenting Portal and presents all things Sleep as part of our Starting Solids and Infant Sleep Masterclass.


Baby Sleeping | How melatonin and cortisol affect sleep

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