How light sets up your mental energy for sustained productivity and resilience

Last week, I explained that we can use three powerful levers to systematically optimize our mental energy around the clock – and be at our best throughout the day and deeply asleep at night.  

Let’s starts with the basics: Our mental energy is controlled by a 24-hour cycle, which is known as the circadian rhythm (circa, meaning "around," and dian, meaning "day"). 

For our Paleolithic ("Old Stone Age") ancestors, it was adaptive to be well-awake (and foraging for resources) during the day and deeply asleep (and safe from predators) at night. As evolution has it, we've inherited this tendency, and evolved to adapt our circadian rhythm to the natural cycle of light and darkness in our environment. 

As a result, bright, blue light wakes us up by firing up our (sympathetic) alertness system in the morning, allowing us to remain alert, focused and productive throughout the day. Conversely, a lack of this type of light makes us sleepy by activating our (parasympathetic) calmness system, which lets us get drowsy, relaxed and eventually able to fall asleep deeply at night.

That's what makes light the most powerful synchronizer (or "time giver") for controlling our mental energy day in and day out. It's only by getting bright, blue light into our eyes early in the day – and avoiding that same light late in the day – that we can get our levels of mental energy to where we want them to be.

The dance of cortisol and melatonin

As it turns out, our light-viewing behavior plays that crucial role by setting off a fascinating dance of two chemicals you've probably heard about before: cortisol and melatonin. Here's why – and how to control that dance to optimize for sustained productivity and resilience.

Our eyes have receptor cells (also called "melanopsin retinal ganglion cells") whose only job is to count photons in our environment. When these cells sense sufficient amounts of bright, blue light around us in the morning (i.e., about 100,000 photons), they communicate with another part of our brain, the circadian master clock (also known as the "suprachiasmatic nucleus"). 

The circadian master clock is a small cluster of some 20,000 nerve cells, which is about the size of a grain of rice and located right above the crossing of our optic nerves. When it gets the signal that there are sufficient amounts of bright, blue light around us in the morning, the circadian master clock signals to every other region of our brain and every organ in our body to switch from night mode to day mode.

It does so by releasing a healthy dose of cortisol into our system. Cortisol has a bad rap and is known as the "stress hormone", which is associated with burnout, depression, and other adverse health issues. However, that's an incomplete picture. 

No matter what we do, all of us get at least one big increase in cortisol once every 24 hours. When that spike occurs early and wears off later in the day, that's good for our productivity and resilience. A spike in cortisol late in the day, conversely, is something we want to avoid, because it's associated with anxiety disorder, major depression, and impaired cardiovascular and metabolic health. If you want to avoid these outcomes and optimize your mental energy to be alert during the day and drowsy in the evening, timing the cortisol peak right is crucial. In that sense, instead of regarding cortisol as a stress hormone, you can also think of it as the "waking-up hormone".

Apart from waking us up by releasing cortisol into our system, there's a second thing that our circadian master clock does when there are sufficient amounts of bright light around us in the morning: It starts a timer for another hormone to be released 12 to 14 hours later. This hormone, melatonin, prepares our bodies for sleep in the evening. Because melatonin loathes light, it's also known as the "hormone of darkness". In the evening, the circadian master clock mistakes bright light for sunlight. Our brain then thinks that it's daytime and, as a result, eliminates the melatonin in our system, making it impossible to get a restorative night of (deep) sleep.

Reverse "dark days and bright nights" 

Here’s the problem with light in today’s world. Unlike our Paleolithic ancestors, most of us go through "dark days and bright nights": We spend an average of 87 percent of our time indoors and rarely get exposed to bright, blue light (sun) during the day. 

Conversely, we get plenty of bright, blue light from our devices at night, which keeps us alert at a time of the day when we want to wind down and eventually fall deeply asleep. 

Reversing this tendency and going for bright days and dark nights is crucial for setting up your mental energy around sustained productivity and resilience. Next, let’s explore how.

Get plenty of sunlight each morning

To set up the dance of cortisol and melatonin, starting your day with sufficient amounts of sunlight into your eyes is vital. Here are three essential tips for doing that.

First, get sunlight before 10 am. Later than that, you enter what scientists call the "circadian dead-zone". During that part of the day, which lasts until the evening, your circadian master clock does not respond to light. If you miss that window, you won't be able to time the cortisol spike and melatonin release right, which adversely effects your mental energy, productivity and resilience.

Second, get outside to view light. Viewing light through a window or windshield will filter out most of the relevant wavelengths. Feel free to wear eyeglasses or contacts, but don't wear sunglasses, because these, too, filter out the wavelength of light you need. And, of course, never look at the sun directly, or in a way that causes you to feel uncomfortable.

Third, adapt the duration to the conditions. The practice is not about consciously seeing the sun, but getting sufficient photons in your eyes. If the sun is out and it's a bright day, go for 5 to 10 minutes. If it's an overcast, dim day, go for at least 10 and up to 20 minutes. As a rule of thumb, aiming for 10 minutes will work fine most days of the year.

Stay away from bright, blue light at night

To not butcher your efforts as the day draws to a close, avoid bright, blue light as much as possible at night. Starting at around 8 pm, eliminate everything in your environment that your circadian master clock might mistake for sunlight. Here are three essential tips for pulling that off.

First, use dimmer lights in the evening. While your brain mistakes bright, blue light in the evening for sunlight, dimmer lights are fine. These include candles if you are able to use them safely. Sounds pretty romantic, doesn’t it?

Second, dim your screens. iPhones and Android phones have built-in features for doing so automatically in the evening. These have names like Night Shift, Night Mode or Dark Mode. On your computer, you can use apps like f.lux, which automatically set your screen to light shades that are less disruptive to your circadian master clock, such as orange or red.

Third, use lower-angle lights. As sunlight naturally comes from above, the receptor cells in your eyes are especially susceptible to overhead lights around you. Hence, use lights that are placed lower in your visual field, such as floor lights and table lamps.

If there is one thing to remember from this post, it's my 10-10-10 rule for optimizing your mental energy for sustained productivity and resilience (as well as deep sleep):

Get 10 minutes of sunlight into your eyes before 10 am and avoid bright, blue light after 10 pm. 

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Until next week,
Christian