Daylight + Physical Health

Summary

Daylight has many important physiological impacts on building occupants. These impacts predominantly manifest through light that enters the eye, though some come from light that is absorbed by the skin. In addition to contributing to eyestrain, daylight influences and properly sets the circadian rhythm, which has numerous impacts on physiology and physical health, both directly and indirectly.

Keywords

Daylight, physical health, circadian rhythm, melatonin, cortisol, physiological health, circadian disruption, sleep

General Physiological Impacts

• Light impacts physical health through two mechanisms: falling on the skin or entering the eye (Edwards 2002).
• Daylight provides the best light for biological function because it has the richest spectral distribution of usable light (Edwards 2002).
• Biological systems impacted by different wavelengths of light include the nervous system, circadian system, endocrine system, and the pituitary and pineal gland (Edwards 2002).

Pain + Healing

• Exposure to daylight provides serotonin, which decreases pain levels and need for medication in hospital patients (Ulrich 2008).
• Daylight has been found to decrease the length of stay for hospital patients, both for mental health issues like depression and physiological health like heart attacks (Joseph 2006, Ulrich 2008).

Comparison of spectral distribution of daylight, LED, and fluorescent light sources. Daylight provides a wider
array of spectral diversity than artificial light sources, highlighting its importance in regulating health. Source: http://www.velux.com/article/2016/daylight-natures-prescription

Headaches, Eyestrain, and Other Issues

• Increased amount of time outside is associated with decreased levels of nearsightedness due to increased light intensity (Aries 2015).
• Proper daylight in buildings reduces the incidences of headaches and migraines (Boyce 2003).
• For children in schools, daylight has been found to offer a number of benefits; e.g. improved eyesight, increased growth, improved immune systems, less cavities (Edwards 2002).
• Workers in windowless factories were found to have higher rates of headaches, faintness and sickness compared to those in factories with windows (Edwards 2002).
• Daylight typically provides adequate light levels free of flicker, but can still result in eye strain due to excessive illuminance, inadequate light levels, glare, shadows, and high contrast between spaces (Boyce 2010).

Light on Skin

• Sunlight on the skin causes skin discoloration, sunburns, with overexposure contributing to risk of skin cancer (Edwards 2002).
• Daylight on the skin is also essential for the production of Vitamin D, which aids in absorption of calcium (Edwards 2002).

Impact on Sleep

• Workers with greater daylight exposure slept an average of 46 more minutes a night than those with less exposure (Boubekri 2014).
• Lack of sleep quality and quantity has a number of negative health outcomes including short-term issues such as memory loss, slower reflexes, diminished attention, weight management through hormones controlling appetite, metabolism, and cortisol (Boubekri 2014).
• Impacts on sleep also affects performance as well though increased fatigue and decreased concentration, leading to greater errors and risk for injury (Boubekri 2014).

Hormone Response

• Hormone regulation is one of the key ways daylight impacts physiological health, affecting melatonin, cortisol and serotonin levels (Edwards 2002).
  • Melatonin, the most important hormone affected by daylight, impacts sleep, mood, body temperature, puberty onset and tumor development (Edwards 2002). High levels of melatonin cause drowsiness and occurs with the absence of light, while low levels correspond with periods of alertness (Edwards 2002, Joseph 2006).
    • Melatonin-depleted blood has been shown to speed the growth rates of breast cancer tumors (Boyce 2010).
    • Low melatonin secretion at night has been associated with increased risks of developing type 2 diabetes (Bedrosian 2016).
  • Cortisol production is stimulated by the presence of daylight and plays an important roll in governing alertness. It also stimulates the immune and nervous system, preparing the body for the day (Edwards 2002, van Bommel 2016).
  • Serotonin is a neurotransmitter that inhibits pain pathways, and levels are increased with daylight exposure and intensity (Ulrich 2008, Aries 2015).

The cyclical pattern of melatonin, cortisol, and body temperature over the course of two days (van Bommel 2004).

Circadian Rhythm

• As seen in the figure below, daylight provides an effective spectral range needed to entertain the circadian system, especially compared to artificial sources.
• The most visible manifestation of the circadian system is the sleep-wake cycle through the secretion of melatonin.
• 

  Circadian spectral sensitivity compared to typical spectral distribution of color temperature light fixtures. Source: https://www.circa-dies.com/circadian-impactful-design/

• The circadian system responds to a different spectrum (colors/wavelengths) of light than the visual system.
  • the blue light of the morning has an alerting affect while the red sky of the evening has a relaxing affect.
• Light levels of 100 lux to 350 lux at the eye (not the work plane) have been shown to suppress melatonin levels
  • Blue-shifted light seems to require less intensity, with levels as low as 40 lux at the eye suppressing melatonin production.
• Exposure to higher levels of light during the day decrease the circadian system’s sensitivity to light later at night, helping prevent circadian disruption.