The dark side of bright nights – A practical guide to Chronotherapy

Hi there, ?

Craving more energy during the day? Struggling to fall or stay asleep at night? Your light exposure—both during the day and at night—may hold the key.

Last month, I?explored how daylight can supercharge your energy. But what about evening light?

This month, I'm?looking at the other side of the equation: how artificial light at night can?sabotage your sleep, leaving you groggy and drained the next day.

This is the second part in a four-part series on how to align your habits and environment with your body's natural rhythms—an approach sometimes called chronotherapy.

A quick note: While the term?chronotherapy is also used in clinical settings to treat?specific sleep disorders, here I’m using it in its broader sense: optimizing biological rhythms, particularly your circadian rhythm.

What's the goal?

Chronotherapy is used to build a stronger circadian rhythm:?the foundation of better sleep, ensuring sharper focus?and a more energized?life. A robust circadian rhythm isn’t just about sleep—it’s the key to unlocking your full potential and helping you thrive at work and in life.

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The way we express brightness: Melanopic illuminance?

When we think about light, we usually focus on how it helps us see. But light does much more than that—it plays a crucial role in regulating our biology, especially our circadian rhythm, or internal clock. This happens through specialized cells in the eye that contain a pigment called melanopsin. This, by the way, was only discovered in 2001 (!!).

But not all types of light have?the same biological impact.

Melanopic illuminance is a measure of how much light affects melanopsin, which in turn influences our circadian rhythm by regulating key processes like alertness, sleep, and melatonin production.

Unlike standard light measurements (like lux), which focus on how bright a space appears to our eyes, melanopic illuminance (measured in mlux) tells us how light affects our body clock.

Why is this important?

• Blue-enriched light has a strong melanopic effect, suppressing melatonin, boosting alertness, and helping you stay awake and focused.
• Dim, warm light has a low melanopic effect, allowing melatonin to rise and helping your body relax, signaling it’s time to sleep.

Understanding how light affects us means we can better control our exposure, to support energy during the day and restful sleep at night.

The dark side of bright nights

Modern life often creates an “extended twilight” at home, confusing our internal clocks. The graph below shows what light intensities—expressed in melanopic illuminance—were measured during the natural sunset (yellow) versus the intensities in people’s homes (blue).

This figure shows how artificial light results in an extended twilight (blue line versus the natural yellow sunset line).

Even very dim indoor light can suppress melatonin by 50% in some individuals. Common culprits include:

• Bright overhead lights.
• Screens from TVs, tablets, and phones.
• LED bulbs, which emit blue light that suppresses melatonin.

When people were exposed to more bright light three hours before bedtime than their average exposure, they were also found to have reduced sleep quality that same night (more time awake in the 90 minutes after they went to bed).

I spoke with Professor Sean W. Cain , a renowned Australian light and sleep researcher who did much of the pioneering work in this space with his?team.

One of the tools they used in these studies was a light meter clipped on people’s clothing to measure their light exposure. He himself had also tried it at home.

He had been shocked by the unexpected light sourced in his home. He quipped, “Even my fridge light at night can sabotage my melatonin!”

Obviously, we don't need to get all paranoid about every single source of light in our home. But, it is?good to be aware?of how “hidden” lights in our house can trick our brain into thinking it’s daytime.

Task:

• Check the brightness you're exposed to after sunset in your home.
• Bathroom lights are often quite bright!
• I personally love the super dimmable light on my nightstand.

You could be much more or less sensitive to light than someone else.

At home, I’ve noticed that I am usually the one who switches off the lights at night. This actually gets?my husband annoyed, as we sit?in too much darkness for his liking.??? For him, the brightness of our living room or bathroom lights at night doesn’t feel like a big deal, while I instinctively reach for the off switches and dimmers.

It turns out that individuals vary a lot in their sensitivity to bright light. When I say a lot, I mean?up to a 50-fold difference.

Imagine this:

?? Person A: Falls asleep easily even after using a bright laptop screen in bed.

?? Person B: Struggles to fall asleep just from the glow of a phone charging across the room.

In research by Professors Sean W. Cain and Andrew J. K. Phillips , some people had their melatonin suppressed by 50% with just the light equivalent of a dim hallway nightlight (3.1 mlux)—while others needed the brightness of a fully lit living room (181 mlux) for the same effect.?

Their?analysis was performed over n?=?42, which gives you an idea of the huge variability. This explains why you might feel restless from a seemingly tiny light source, while someone else has no idea why you care so much. ??

World Sleep Day is coming up!

This March invest in?performance & well-being!

March is Sleep Awareness Month, and World Sleep Day (March 14, 2025) is the perfect opportunity for organizations to invest in better sleep, better performance, and better well-being.

I’m offering a limited number of exclusive engagements for companies looking to take sleep seriously—without complex programs or major time commitments. ?

?? Our online tool delivers science-backed, personalized chronotherapy tips to help employees improve their energy, focus, and health. All tips are tailored to their chronotype. ?? Easy to implement, high-impact results—happier, healthier, and more productive teams.

?? Want to bring this to your organization?

Send a message to [email protected] to learn more.

A personal reflection: Glamping and better sleep

Every summer, my husband and I take our kids “glamping” in the Netherlands—a bit of a nostalgic nod to my childhood. ??

Spending long days outdoors, even under cloudy skies, helps us wind down naturally. By 9 PM, we’re usually ready for bed, enjoying some of the best sleep of the year. I always wake up refreshed and start my day with a run or mountain bike ride in the nearby forest and dunes.

This isn’t just anecdotal; science backs it up?????

Studies show that people exposed to 30 hours of outdoor light a week fall asleep nearly two hours earlier than those with 10 hours or less a week.

Do you know how much light you are getting during the day?

Although a device like the one used by Sean Cain is the most accurate, measuring your light exposure is surprisingly easy (and quite fascinating!) with a smartphone.?Many light meter apps are available for free, and while a light diffuser improves accuracy, simply holding a piece of white paper in front of the camera works well.

Of course a proper light meter will be more accurate, but to get a rough sense of brightness, it works well enough.

Typical indoor lighting ranges from 100 to 1,000 lux, depending on the environment.

I tested this at home and found that my west-facing office gets around 2,100 lux on a sunny day?and 800 lux on a cloudy day—decent, but nowhere near what you get outdoors.

For comparison, some outdoor values:

• Cloudy day outdoors: typically 3,000 to 30,000 lux
• Shaded outdoors on a sunny day: ~10,000–25,000 lux
• Direct sunlight on a bright day: over 100,000 lux!

These large differences highlight the significant contrast between indoor and outdoor light exposure. Even on a cloudy day, stepping outside provides substantially more light than typical indoor environments, which is crucial for regulating your circadian rhythm.

Just so you know, this?all depends?on factors like time of day,?location, cloud density, and atmospheric conditions.

The more daylight exposure you get—especially in the morning—the stronger your internal clock, making it easier to fall asleep and wake up feeling refreshed.

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My best chronotherapy tips

Want to dive deeper into optimizing your light exposure for better sleep, mood, and focus? Download my free chronotherapy and light guide:

• Practical tips for every part of your day.
• Tools and apps to track your progress.
• Science-backed strategies to live in harmony with your circadian rhythm.

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Try it out yourself!?

Why not track your light exposure for a week? Experiment with morning and evening light routines?

Share your insights in the comments below this newsletter.?I’d love to hear about the tools or techniques you’ve tried.

Stay tuned for more on how food can help or hurt your circadian rhythms next month. Until then, I wish you great sleep!

Dr. Els van der Helm