How Light and Darkness Affect our Sleep


Have you heard the term “nocturnal?” Of course you have! Animals that sleep in the day and do most of their activities at night; bats, owls, and raccoons being some familiar examples. Likely you are not as familiar with the term diurnal (I wasn't previously), but that's what you are... I hope (unless you work nights). Humans generally work, play, eat, live during the day and rest during the night. (Interesting side note, around 80% of mammals evolved to sleep during the day.)

One of the benefits of being diurnal, is something called a 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 a clock, it has a LOT of moving parts, only instead of gears and springs, it’s made up of stimuli and hormones. 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 who’s having a hard time sleeping, you have no doubt heard about melatonin, it may have even been recommended to you as a supplement by a well meaning friend. 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 a 11-12 hour stretch of glorious sleep, you can thank their pineal gland for firing up those melatonin pumps. But you can also thank the daylight, because exposure to the sun stimulates melatonin production. Getting outside during the day really does help them sleep better at night!

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, hopefully for a long, restful night.

Come morning, the blue light from the sun starts to permeate the thin skin of our closed eyelids, signaling the brain that it’s time to get back into gear. So now our brain is going to help us get out of bed by telling our adrenal glands to pump out some cortisol.

Cortisol isn't only a dirty word. Often, it gets a bad rap because people associate it with stress. The truth is, cortisol is a very beneficial hormone. It regulates metabolism, blood pressure, 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.



This intricate dance between light and dark, cortisol and melatonin, awake and asleep, evolved over an incredibly long time, and it worked really well until... artificial light was introduced to humans. Before that, we relied exclusively on fire, and fire emits very little blue light which is harmful to melatonin. Conversely, light bulbs, depending on their hue, emit quite a bit of blue light. And TVs, LEDs, computer monitors, iPads, smartphones, and all of those other screens that surround us in the modern day, absolutely flood our eyes with it. Unfortunately, all of that blue light coming at us in hours when we would normally be 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 turn off those really 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 real dark. Can’t see your hand in front of your face kind of dark. Blackout blinds can also be a game changer, especially if you live somewhere where the days get exceptionally long in the summer.

Of course, great sleep is like your favorite cookie recipe. Each ingredient plays a key role in the quality of cookie. You need all the ingredients to work together to make the best version. Just like with sleep, there are many wonderful ingredients that add to a great night sleep, all of them important. Consider darkness to be the sugar, without it, the cookie won't be nearly as good!

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