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, our bodies pay a high price. We’ve already mentioned how chronically elevated cortisol is disruptive, causing premature aging and visible belly fat deposits in chronically stressed people, both sedentary folks and recreational athletes alike. Many aspects of modern life serve to elevate cortisol and/or suppress melatonin at inappropriate times, be it many of the common low-calorie diets, badly timed and/or excessively long fasts, excessive exercise (think: Jill’s excessive HIIT regimen), shift work (and the associated circadian rhythm disruption), the stress of overscheduled lives, or even the anxiety of scrolling through social media, a common pre-bedtime routine for many (note: feeling pressured to project the right image on social media, or trying to equal or surpass the projected images of others, isn’t optimal for inducing a blissful nighttime repose). Comparison isn’t relaxing, and social media has a comparative aspect programmed right into it. All of these things serve to elevate cortisol as a part of our stress responses to life’s daily pressures.
With a basic sense of the underlying neurology in place, we’re now in a much better position to understand the devastating impact that a lack of daytime bright natural light exposure can have. Recent research has suggested that spending too much time in relatively low-light rooms could be changing the way our brains process information and impairing the growth of new neural connections. “Are Dim Lights Making Us Dimmer?” read the headline of one report I reviewed.21 Our increasingly indoor lifestyles are also thought to be behind the global nearsightedness (myopia) epidemic, where up to half of young adults in the United States and Europe, and upward of 90 percent of Asian teenagers are affected—a massive change from a half-century ago. The strongest environmental risk factor for this large-scale loss of visual acuity across our populations of teenagers and young adults: the lack of bright natural light exposure associated with being indoors most of the day.22
Perhaps the best example of the impact a lack of bright light exposure can have on our psychological health comes from looking at those who suffer from the winter blues: seasonal affective disorder (SAD) and its milder variant, subsyndromal seasonal affective disorder (SSAD). SAD/SSAD is a form of depression that’s related to light changes in the seasons, most commonly autumn and winter, but is known to also occur in spring and early summer. There seems to be a clear link between light exposure and a change in our mood, outlook, and well-being.
We know that many animals change their behaviors in the winter months as the light wanes, with some going into complete hibernation. A decline in serotonin levels with the reduced light exposure (both duration and intensity) and a concomitant increase in daytime melatonin levels, often in conjunction with dietary factors such as an insufficient specific amino acid intake, is at the heart of the winter blues we can often feel ourselves slip into.23 Living far from the equator appears to be a key risk factor for experiencing seasonal affective disorders, further supporting the suggestion that changes in natural light exposures are fueling this phenomenon. Indeed, in high-latitude regions such as Finland and Alaska, around one in ten people are affected by SAD, and one in four by SSAD.24 Compare this to fewer than two in one hundred people in Florida.25 Seasonal mood disorders are also more pronounced in regions that suffer cloudier winters, further reinforcing the notion that light plays a key role in our mood and feelings of well-being.
Symptoms of winter-onset SAD include low energy levels, tiredness, cravings for high-carbohydrate foods (driving increases in body fat), sleeping problems, difficulty in concentrating, feelings of hopelessness or worthlessness, and suicidal ideation. Rather than depression, summer-onset SAD, driven by excessive light exposure (such as might be experienced during the “white nights” of high-latitude countries in the summer months), is more likely to be characterized by anxiety and mania. The easily overstimulated, hyperactive, and obsessive-compulsive tendencies that accompany summer-onset SAD are more commonly associated with unhealthy weight loss rather than weight gain.26 These extremes give us insight into the effects of light—too little, too much, poorly timed—on our mood and behavior. While both winter- and summer-onset SAD may represent extremes, most of us function and experience variances in our moods along a continuum of light exposures. It’s hard not to observe that rates of depression and anxiety are increasing as our light and dark exposure patterns are perhaps at the most extreme they’ve ever been in human history.
Insufficient bright light exposure not only leads to low serotonin levels and poorly timed cortisol pulses, but as bright light exposure also catalyzes dopamine synthesis, a lack of well-timed bright natural light can also lead to low dopamine levels.27 Dopamine is our motivation, pleasure, and mood neurotransmitter, and is part of a system all too readily hijacked by modern life. Think about the overabundance of drugs, alcohol, pornography, gambling, and processed food. What propels us to seek out sunlight, and what’s responsible for the euphoric feelings we get once we are in it? Dopamine. Knowing this, it should also
