The Regulating Properties of Sunlight and Darkness
I grew up backpacking and camping in the High Sierra mountains of California. I remember going to bed shortly after dark and waking to the grey dawn. This was before the advent of devices and I only had a small flashlight in my tent, so there wasn’t much to do after sunset. We would gather for a short time around a campfire or curl up in a sleeping bag with a good book. I slept well in that setting.
In contrast, modern conditions hijack a good nights rest: It can be as simple as the use of overhead lights or checking email before bed, watching tv or reading from an illuminated tablet. In our industrialized culture, the constant exposure to these bright lights interrupt a delicate and ancient rhythm of hormones. At night cortisol, produced by the adrenals to help you wake in the morning, decreases and melatonin is produced making you sleepy. Exposure to blue and white light after sunset causes melatonin levels to decrease and cortisol to increase making it harder to go to sleep and stay asleep and generally jeopardize sleep quality and quantity. Most of us spend 90% of our time inside. And this can have consequences on our health.
Deep inside of your brain, within the anterior hypothalamus, lies a dense congregate of neurons elegantly named the Suprachiasmatic Nucleus otherwise known as the master circadian clock. According to Dr. Andrew Huberman, of the Stanford School of Neurobiology and Ophthalmology, the single-celled organisms that we evolved from, were able to sense color before pattern in order to establish the circadian rhythm. Therefore our access to natural light and to darkness determines our brain’s ability to assess where we are in time, if it’s day or if it is night; when it’s time to wake up and to go to sleep. New research also demonstrates the powerful role that both sunlight and darkness play in both mental health outcomes and in mood.
Scientists have for years correlated day length and mental health. Seasonal Affective Disorder (SAD) arises in many people, even those not living close to the poles, during the darker months. In a recent cohort study of 85,000 people, the largest study on light exposure, published in peer-reviewed journal Nature Mental Health, researchers were able to determine that greater night-time light exposure increases risk of psychiatric disorders and poor mood and, independently, greater daytime light exposure reduces risk of psychiatric disorders and poor mood. In those exposed to bright light at night, depression increased by 30%. Conversely, those exposed to bright light during the day experienced a 20% decrease in depression.
Light level intensity is measured in Lux. The sun at noon produces around one hundred thousand lux and on a very bright day can be over three hundred thousand lux. Most indoor environments with overhead lights produce approximately 4-6 thousand lux. Lights engineered to treat seasonal affective disorder (SAD) produce about 10 thousand lux. It is easy to see, comparing these levels, that even with a full day under bright indoor lights, it would be difficult to meet our needs for light exposure. And if you live or work in an environment with a large amount of natural light you still need to head outside as windows scatter most of the relevant light waves.
Four important points that have influence on circadian rhythm and mental health outcomes include:
Exposure to the dawn light
Exposure to bright daytime light
Exposure to sunset light
Exposure to darkness at night, ideally for 8 hours.
Cells in the neural retina, in the back of the eye, reside in the bottom 2/3 of the retina and are responsible for looking up to gather light. They respond to two different types of light input: Short and long wave light. Short wave light is blue and purple and longer wave lights are orange, pink and red. At sunrise and sunset the sky is saturated with both blues and oranges, pinks and reds. Viewing this contrast causes these cells, called ganglion cells, to send signals to your hypothalamus which then sends out chemical signals to the body: An alert that it is day or night. They can also trigger negative affect via the release or suppression of neurotransmitters like dopamine and serotonin, which steer mood. Retinal sensitivity is lower in the morning hours, so the need for photon exposure is greater during that time. Greater retinal sensitivity at night makes it easier to interrupt a healthy bed time through artificial light exposure. However, exposure to natural light at sunset halves the melatonin suppression and partially offsets the negative effects from night time light exposure.
We humans are social animals and our ancestors lived in multi-generational groups. But these days many of us live alone. Because of this, extended time indoors is also socially isolating. Consider meeting up with a friend for a daily walk, rain or shine, build a routine around spending a few minutes outside during the early morning and sunset hours and create a dark and cozy atmosphere in your home in the evening and night time. Your brain will thank you for it!