Sleep Series: Underappreciated Importance of Daylight

The Sleep Economy is a $450 billion dollar market and yet, we’re still not sleeping!

We live in a sleep-obsessed culture; we buy smartwatches and rings to track sleep quality, take CBD and Xanax, and buy overpriced "smart" mattresses. Sleep is becoming such a booming wellness trend, and we are even seeing a rise in orthosomnia, a condition where anxiety over sleep tracking causes sleep problems. With an abundance of sleep solutions, why do we remain in a sleepless epidemic with around 1 in 3 of us sleeping poorly and 1 in 10 having regular insomnia? The answer lies in what's forgotten, these sleep "solutions" have ignored our circadian biology; our built-in 24-hour sleep-wake cycle that has evolved over millions of years as a response to the earth's rotation.

Circadian Clock

The solution to sleep revolves around light. As discussed in my last post, we’re ruled by an inner clock that adapts to light. The bedrock of circadian science is that exposure to regular light-dark cycles provides the daily “time cues” needed to reset our circadian clocks every single day. This time clock not only determines how well we sleep, but impacts our overall cellular health. We need the sun’s bright blue light in the day to be alert and active, and we need dark to kick-start our brain’s sleep mode and recovery.

This image below shows the correlation of time of day and certain biological functions.

Let There Be Light

The solution to a better night sleep could be as simple as more light exposure during the daytime, and less light exposure in the evening. While there’s been an abundance of attention on limiting evening screen time and the the role of blue light in melatonin suppression, there has been almost no attention given to our lack of daytime light exposure.

The illuminance of light in the eye is measured in lux — a complex measure of the quantity of light. For adults, it takes about 350-500 lux to trigger the circadian system to promote significant daytime wakefulness and, in turn, promote nighttime sleep. Our bodies have evolved to be in bright light during the day optimally absorbing between 1,000-10,000 lux to keep a regular circadian rhythm. Since most of us spend the majority of our time indoors, we're only getting around 100 lux from most indoor lighting. In other words, there’s at least ten times less light inside than outside, even on a cloudy day. What if the lack of quality daytime light was the cause of the sleep epidemic? A recent study showed daily exposure to sunlight each morning helps to re-align the circadian rhythm so that people get better, more regular sleep.

Does Blue Light From Screens Really Ruin Sleep?

Before the invention of the light bulb, we relied on the moon and fire to see after dark. Times have definitely changed – now we keep bright lights on in the house after dark and binge Netflix into the wee hours. How much is this night-time light exposure really affecting our sleep? With the invention of blue-light blocking glasses and "dark mode" on our devices, we'd think a lot, right?

Turns out the negative effects of blue light at night can be minor so long as you're getting the right amount of light during the day.

10:1 Ratio

We know it takes about 350-500 lux to trigger the circadian system to promote daytime wakefulness. We also know that at full brightness, a TV or phone screen delivers anywhere from 1 to 37 lux. So as long as you're getting sufficient light during the day, 37 lux of light at night is inconsequential.

What really matters is the ratio of daytime light intensity and duration to the amount of indoor light intensity and duration at night. So if you spend all day in a dim room (100 lux), then stare at a bright screen (37 lux) after dark, it could be disruptive to your sleep and wake patterns. You could think about like a 10:1 ratio; as long as you're getting 10 times the light exposure during the day than at night, you'll be sleeping like a baby.


So here's the Hidden Lever: Aim for a high ratio of light during the day to light at night. Light exposure during the day could have a protective effect against screen light in the evening.

Stay tuned for Part 3 on methods to improve your sleep routine and the future of the sleep economy.

Until next time,

Justin