Sunlight; one of the most underrated and often ignored components to a healthy lifestyle. People in developed and industrialised countries spend an average of 93% of their time inside, away from the sun’s invigorating and balancing ultraviolet light [3]. Most of us work jobs and study inside, live, eat and sleep indoors and are increasingly spending our free time within the confines of buildings. This may feel comfortable and adheres to our habits but is keeping us from an array of beneficial effects that sunlight can provide.
Weather and sunlight can create immediate and lasting impacts in mood and cognition including; greater feelings of happiness resulting in increased helping behaviour and empathy [1], improved cognitive performance and flexible thinking styles [3], lower self-reported depression scores and increased vitamin-D levels [4, 8] and restricts the conversion of serotonin (hormone that contributes to feelings of happiness, contentment and relaxation) into melatonin (hormone that regulates sleep) [1,6].
Exposure to sunlight is the strongest cue for controlling the human circadian timing system (24-hour cycle of sleeping/waking patterns, hormones, energy levels, among many other cycles). Sunlight strongly controls our internal clock and keeps us synchronised with the external day and night [9]. Bright light in the morning, shortly after waking, will shift our internal clock, allowing us to get to sleep earlier. Whereas bright light in the evenings will push our clock forward, making it harder for us to get to sleep until the late hours of the night [9].
The use of screens in the late evening, particularly white and blue light(phones, laptops, televisions, or even bright home lighting),will modulate the suprachiasmatic nucleus (the ‘internal clock’ of the brain, which is sensitive to light) and prevent the conversion of serotonin to melatonin [1,6]. While this effect is optimal during the daytime to stay awake and in good mood, at night it will prevent us from calming and slowing down into a state more useful for sleep.
As we bask in the sun’s powerful ultraviolet (UV) radiation, several critical mechanisms are kicked into motion, enhancing and regulating our mood and central nervous system (CNS). Most of us are aware that sunlight increases vitamin D levels, which contributes to bone density and health. What isa hugely overlooked function of vitamin D is its ability to serve as a powerful antidepressant and anti-inflammatory factor. VitaminD has a direct effect on the CNS through the vitamin D-receptors, which increase production of the anti-inflammatory cytokine Interleukin-10 (IL-10)[4]. It is well-known within scientific research that inflammation plays an important role in symptoms of depressive and anxious behaviour. Sunlight, and vitamin D are therefore a powerful anti-inflammatory agent that should be used to combat depressive symptoms every single day if possible.
Some populations (northern countries like Finland, Sweden, Norway) are unable to achieve adequate sunlight during the winter months. Therefore, the UV radiation that is needed to synthesis vitamin D, along with all the additional anti-depressant advantages are simply impossible to achieve. This time of year results in a regular spike in depressive symptoms and is known as seasonal affect disorder (quite aptly abbreviated to SAD). Patients withSAD will experience normal depressive behaviours (lack of energy, over-sleeping, lack of pleasure and increase carbohydrate cravings) during the fall months, and usually full remission during the spring time [5].
If you live in an area that proves to be too difficult to achieve regular sunlight exposure, then light-box therapy is a proven therapy to improve SAD symptoms. Over 70 controlled studies have proven the effectiveness of light therapy, also proving to be as effective as anti-depressant medications (without all the possible, nasty side-effects) [5]. The aim for light-box therapy is 30 minutes of 10,000 lux or brighter light directly in front and above the eyes within 30 minutes of waking. These guidelines are also applicable for regular sunlight use [5].
On top of all the physiological mechanisms mentioned in this article, sunlight exposure and spending time outdoors is a great ‘two birds with one stone’ activity to deal with depressive mood. While we are outdoors, we are often participating in activity where we are distracted from our regular, ‘at rest’ mindset where depressive thoughts can ruminate and cycle over and over in our minds. This is incredibly important to break free from patterns that contribute to depressive mood. Also, mood maintenance hypothesis states that; “individuals seek out positive activities while in a happy mood in order to maintain or elevate mood. This tendency may become over learned and automated” [2].
Considering the benefits, cost and availability of sunlight exposure, it may be a method that can be implemented into all of our lives immediately and at no risk of negative side-effects (exempting sun-burn). It is important for all of us despite the fast-paced tendencies of modern life, to take much-needed, quality time outdoors.
References:
1. Devaraj, S;& Patel, P. C. (2017). Taxicab tipping and sunlight. PloS one, 12(6), e0179193.
2. Handley, I;Lassiter, G;Nickell, E;& Herchenroeder, L. M. (2004). Affect and automatic mood maintenance. Journal of Experimental Social Psychology, 40(1), 106-112.
3. Keller, M;Fredrickson, B;Ybarra, O;Côté, S;Johnson, K;Mikels, J;... & Wager, T. (2005). A warm heart and a clear head: The contingent effects of weather on mood and cognition. Psychological science, 16(9), 724-731.
4. Knippenberg, S;Damoiseaux, J; Bol, Y;Hupperts, R;Taylor, B. V;Ponsonby, A;... & van der Mei, I.(2014). Higher levels of reported sun exposure, and not vitamin D status, are associated with less depressive symptoms and fatigue in multiple sclerosis. Acta Neurologica Scandinavica, 129(2), 123-131.
5. Lam, R. W;Levitt, A. J;Levitan, R. D;Enns, M. W;Morehouse, R;Michalak, E;& Tam, E;(2006). The Can-SAD study: a randomized controlled trial of the effectiveness of light therapy and fluoxetine in patients with winter seasonal affective disorder. American Journal of Psychiatry, 163(5), 805-812.
6. Levandovski, R;Pfaffenseller, B;Carissimi, A; Gama, C;& Hidalgo, M;(2013). The effect of sunlight exposure on interleukin-6 levels in depressive and non-depressive subjects. BMC psychiatry, 13(1),75.
7. Paul, M;Love, R;Hawton, A; Brett, K;McCreary, D;& Arendt, J. (2015). Sleep deficits in the High Arctic summer in relation to light exposure and behaviour: use of melatonin as a countermeasure. Sleep medicine, 16(3), 406-413.
8. Xu, Y; Qi, J;Yang, Y;& Wen, X. (2016). The contribution of lifestyle factors to depressive symptoms: A cross-sectional study in Chinese college students. Psychiatry research, 245, 243-249.9. Zeitzer, J;Friedman, L;& Yesavage, J;(2011). Effectiveness of evening phototherapy for insomnia is reduced by bright daytime light exposure. Sleep medicine, 12(8), 805-807.
