cities, such as New York, the surrounding buildings can block much of the available sunlight and cast significant shadows. But you will still see people wearing sunglasses, blocking the light further still. In stark contrast to our preindustrial human ancestors, who would have awoken slightly before sunrise and who would have spent their mornings actively exposed to bright natural light (sans Ray-Bans), we modern humans might be lucky to get thirty to ninety minutes of bright light exposure during summer mornings, often filtered by sunglasses and/or UV-tinted windshields, before we scurry back indoors, missing out on not only the peak brightness of the day, but also a significant duration of bright light exposure.
To understand our (lack of) light exposure further, we need to understand a couple of measurements: lumen and lux. Lumen is a measurement of light intensity (brightness) taken at the source of the light itself. As light travels away from its source, it scatters into the surrounding area and its intensity changes. Think about a bright LED flashlight shining directly into your eyes versus being fifty feet from it. Lux takes the lumens of a light source and factors in the area over which the light spreads, giving an indication of how bright, for example, a light source is in a particular room.
To give you some scale and perspective on lux readings, the light on a clear day in the summer can exceed 100,000 lux; on a dark and cloudy day in the same outdoor space, it can be as low as 1,000 lux. Full daylight but indirect sunlight can measure 10,000 lux. At night, with a full moon, it would be less than 1 lux. Sunrise or sunset on a clear day is around 400 lux. Now let’s compare these natural light scenarios to some typical artificial lighting. Bright office lighting comes in around 300 to 500 lux (comparable to sunset). An office hallway might be around 100 lux. A very brightly lit home living space might come in at a similar reading to the office but is more likely to be under 100 lux. That means that bright natural daylight is one hundred to one thousand times brighter than our typical indoor lighting. That’s a huge difference.13
Linda Geddes, author of the book Chasing the Sun: The Astonishing Science of Sunlight and How to Survive in a 24/7 World, set about on an interesting light experiment in conjunction with sleep researchers from the University of Surrey (UK). Following the same argument that I make here, that our preindustrial ancestors lived and slept in tune with the light and dark cycles of the natural world, Geddes set about to live for four weeks with as little exposure to artificial light after sunset as practicably possible (in the context of having a career and family to manage). Part of her experiment involved measuring the intensity of her light exposure during the day.14
On one particular morning, sitting in the park after dropping her children off at school, Geddes measured the light intensity at 73,000 lux. She took another reading at her desk once she arrived in her office, 120 lux. That is, the light in the environment she would be exposed to for much of the day was about one-fifth of the light intensity she might get immediately after sunset, and only a tinier fraction of what she would get if she were outside. Even moving to a desk closer to a window where it was sunnier, the light intensity was 720 lux—still over one hundred times less than her light exposure in the park earlier that morning.
Across the duration of Geddes’s four-week experiment, where she attempted to get more light exposure during the day, her average exposure between 7:30 a.m. and 6:00 p.m. was just under 400 lux in the first week of the experiment and as low as 180 lux in the second (but these were still increases from her preexperiment baseline of 128 lux). The experiment did take place in the middle of a UK winter when sunset occurred at 4:00 p.m. Nonetheless, the magnitude of the difference in light intensity between indoors and out is clear, being in the order of at least one hundred times less for the indoor environments, irrespective of the season.
Inspired by Geddes’s experiment, I purchased a light meter from an electronics shop and began tracking the brightness of the light in the various settings I would find myself in on a daily basis. Without exception, and irrespective of the weather or cloud cover, outdoor light was always at least ten times brighter than the indoors, and more often one hundred times brighter. Early in the morning, the light in my house might be 100 lux, while outside at the same time, in indirect light, it was 1,000 lux. At my local café, it would be 300 to 400 lux seated indoors, and 30,000 to 40,000 lux seated outdoors. Conversely, at night, I recorded outdoor readings of less than 1 lux, while indoors, with the bright, blue-light-emitting artificial lights on, I would get around 200 lux. Switching the main lights off and using a low-wattage incandescent lamp brought the brightness down to under 10 lux.
As these experiments demonstrate, we’re not getting the bright light we need during the day. As a result, we’re confining ourselves to chronic summer sleep. We are in effect living in the weak winter light of the high latitudes during the day. Our indoor lives send the message to the light-sensitive part of our brains that it’s dawn or dusk most of the time. With our increasing bright artificial (blue) light exposure after sundown, quite literally at the push of a button and flick of a switch, we switch to high-latitude summer light in our evenings. No wonder our brains don’t know whether to be alert or asleep a lot of the time! We’re sending really inconsistent and incoherent light signals. In the following chapters, we’ll discuss how we’re in perpetual summer mode when it comes to our diet, physical movements, and social interactions. Playing out summer sleep patterns throughout the entire year is just as unnatural and damaging to our health. Returning to the natural oscillation of the light/dark cycle on both a daily and seasonal basis is a vital and often overlooked route to better health and wellness.
It’s All About the Neurology, Baby
How exactly does insufficient exposure on a regular basis to bright light or darkness disrupt our bodies’ physiology? Let’s take a closer look at the basics of our light biology and the circadian and diurnal rhythms mentioned earlier in the book. Natural daylight—light from the sun—contains the full spectrum of light, including invisible ultraviolet light at one end (which is involved with vitamin D production in the skin, tanning, and, when overexposed, sunburn) and invisible infrared light at the other (which gives us the sensation of warmth and heat). It is a specific segment of this spectrum—the shorter wavelength blue-light spectrum—that is involved in signaling and synchronizing our sleep-wake cycles. The presence of blue light stimulates our transition to daytime physiology and wakefulness; its absence, the transition to nighttime physiology and sleep. It is this diurnal light-dark cycle that sets the endogenous circadian rhythm described in chapter 1.
Receptors in our eyes (called intrinsically photosensitive retinal ganglion cells, or ipRGCs) that make up part of our circadian rhythm system contain a vitamin-A-derived protein pigment, melanopsin, that is sensitive to intense blue wavelength light, the kind we get from sunlight not long after sunrise.15 When morning light stimulates these receptors, it activates neural pathways and hormonal responses that help increase our wakefulness, alertness, and body temperature. The light literally wakes up and primes our body for the day. That light also suppresses melatonin. As the intensity of blue light declines toward the end of the day, being replaced, at first, by visible red light (such as is seen at sunset, or emitted by firelight), and eventually full darkness, melatonin secretion increases, initiating our sleep processes and helping us to, hopefully, fall asleep. A key part of our brain, the suprachiasmatic nucleus (SCN), or the master body clock, coordinates and synchronizes these light- and dark-triggered circadian rhythm events day after day. We are exquisitely tuned to the presence or absence of light, and we have very specific physiological responses to differing light triggers.
When it comes to sleep, many people focus on melatonin as our primary “sleep hormone” because of the association of low levels of melatonin and poor sleep architecture (the cyclical pattern during our sleeping hours). When you lie awake at night, or have restless sleep, low melatonin is usually a big part of that. This often leads us to hit up the local drugstore or Amazon.com in search of melatonin supplements, which we use as either an everyday sleep aid or to
