An often overlooked component of a paleolithic lifestyle is spending time outside.  While fresh air is invigorating, this is really about getting sun exposure.  Getting enough sun exposure is vital for our health.  You probably already know that our bodies make vitamin D in response to exposure to ultraviolet light.  (Yes, too much exposure to intense ultraviolet light can cause DNA damage in your skin cells and increase your risk of skin cancer, but as long as you aren’t getting sunburns, exposure to ultraviolet light is not only safe, it’s essential for your health.)  Vitamin D is a steroid hormone that controls expression of more than 200 genes and the proteins those genes regulate.  Vitamin D is essential for mineral metabolism (it regulates absorption and transport of calcium, phosphorous and magnesium) and for bone mineralization and growth.  Vitamin D is also crucial for regulating several key components of your immune system, including formation of important anti-oxidants.  Very importantly, Vitamin D has recently been shown to decrease inflammation and may be critical in controlling auto-immune and inflammatory diseases.  Vitamin D is also involved in the biosynthesis of neurotrophic factors, regulating release of such important hormones as serotonin (required not only for mental health but also for healthy digestion!).  Vitamin D helps control cell growth, so it is essential for healing.  Vitamin D also activates areas of the brain responsible for biorhythms.  Scientists continue to discover new ways in which Vitamin D is essential for human health; for example, Vitamin D may prevent cancer. 

Vitamin-D production from ultraviolet light exposure is not the only important aspect of sun exposure.  Cells throughout the body, including the skin and eyes, are sensitive to blue light from the sun, which is strongest in the morning.  When special cells in the retina of the eye are stimulated by sunlight, they directly affect the pituitary gland and the hypothalamus region of the brain.  The hypothalamus is responsible for circadian rhythm (your body’s internal clock) and regulation of hormones and the nervous system.  Proper regulation of circadian rhythm is crucial for quality sleep, stress management, and the cyclic pattern of expression of so many hormones in the body.  One important circadian rhythm hormone, produced by the brain’s pineal gland and regulated by sunlight, is melatonin.  In addition to being critical for quality sleep, melatonin is a powerful antioxidant, is important for intestinal function, and can help prevent depression.  The hypothalamus and the pituitary gland influence the adrenal glands, which control cortisol production.  These important effects on brain activity, which increase alertness, improve cognition, and boost mood and vitality, are all independent of Vitamin-D production.  So, while taking a Vitamin-D3 supplement is very helpful when the sun is scarce in the winter months (or if you do shiftwork or face other challenges to getting out into the sun), it can’t replace the huge range of health benefits of just plain old getting outside. 

How much time should you spend outside? It depends on your personal body chemistry, how much skin is exposed, what time of day it is, what time of year it is, clouds and air quality, and where you live.  Aim for as much as possible (taking care not to get a sunburn!), but at least 15 minutes most days is a good rule of thumb.