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 stave off jet lag when traveling. Melatonin, however, is most potent when produced as a downstream product of our daytime physiology, specifically, its precursor, serotonin.
The neurotransmitter serotonin, which I’ve described as characterizing the fall season, is important to us every day. It helps regulate our mood, appetite, memory and learning, and, you guessed it, sleep. Exposure to bright early morning natural light boosts serotonin production (in conjunction with the amino acid tryptophan and other vitamins and minerals consumed as part of a protein-rich breakfast), providing the raw materials for the melatonin required for our nighttime physiology.16 The converse is also true. The low melatonin leads to poor sleep train of thought is an oversimplification. In fact, low morning light exposure plus a low protein intake (leading to low tryptophan and cofactor intake) leads to low serotonin production, which leads to low melatonin production, which leads to poor sleep. If we don’t supply the building blocks and bright daytime light triggers for serotonin synthesis, we won’t have adequate serotonin to convert into melatonin. You know that pleasantly relaxed, tired-but-not-frazzled feeling you have after a long day of hiking or playing at the beach? And you know how you often naturally want to head to bed fairly early on those days, maybe after sitting around a bonfire with your friends or family, and how you usually sleep really well that night? Yeah, that’s the effect of lots of bright daytime light, lots of serotonin production, and lots of melatonin availability. That’s the normal experience of the effect of natural light on your physiology. At the same time, we all now know that nighttime, blue-light screen time suppresses melatonin, so we could still undermine a perfectly good day of natural light exposure with unnatural light after dark.17 This is so common that experts have a name for it: light-induced melatonin suppression, or LIMS.
Melatonin’s role in sleep is just the beginning. Melatonin performs a variety of functions in the body, making it indispensable for a long and healthy life. Melatonin has antioxidant properties, meaning it fends off damaging free radicals in our bodies, thus protecting us from a range of maladies, from migraines to deadly neurodegenerative disorders like Alzheimer’s.18 Melatonin also enhances our immune systems, and appears to be protective against a variety of cancers, especially breast and prostate cancer.19 Melatonin receptors are present in many parts of the body, including the blood vessels, ovaries, and intestines. Melatonin appears to help regulate reproductive hormones in women through its interactions with the ovaries and pituitary gland. Melatonin even influences the timing, frequency, and duration of menstrual cycles. Melatonin, in nonhuman mammals at least, also helps to cue mating.
Serotonin is the daytime neurohormone, but don’t think of it as the functional polar opposite to the nighttime melatonin. Cortisol plays that role. Like serotonin, cortisol production is stimulated by exposure to bright light such as sunlight and is the primary hormone for getting us awake and going in the morning.20 It’s healthy and normal to have elevated cortisol levels in the early to midmorning, but not beyond. We all require a strong, well-timed cortisol rhythm, where cortisol rises sharply from early in the morning (just prior to sunrise), peaks around midmorning following bright sunlight exposure (while melatonin is low), then drops away over the remainder of the day and into the evening (as melatonin begins to rise once again in the absence of bright light). When the rhythmic interplay between cortisol and melatonin is disturbed in any way, particularly chronically,