The Role of Daylight for Humans: Gaps in Current Knowledge.

Céline Vetter,Klaus Martiny,Katharina Wulff,Thomas Kantermann,Steven A Brown,Mirjam Münch,Kenneth P Wright,Debra J Skene,Anna Wirz-Justice,Oliver Stefani

doi:10.3390/clockssleep2010008

Abstract

Daylight stems solely from direct, scattered and reflected sunlight, and undergoes dynamic changes in irradiance and spectral power composition due to latitude, time of day, time of year and the nature of the physical environment (reflections, buildings and vegetation). Humans and their ancestors evolved under these natural day/night cycles over millions of years. Electric light, a relatively recent invention, interacts and competes with the natural light–dark cycle to impact human biology. What are the consequences of living in industrialised urban areas with much less daylight and more use of electric light, throughout the day (and at night), on general health and quality of life? In this workshop report, we have classified key gaps of knowledge in daylight research into three main groups: (I) uncertainty as to daylight quantity and quality needed for “optimal” physiological and psychological functioning, (II) lack of consensus on practical measurement and assessment methods and tools for monitoring real (day) light exposure across multiple time scales, and (III) insufficient integration and exchange of daylight knowledge bases from different disciplines. Crucial short and long-term objectives to fill these gaps are proposed.

Highlights

IntroductionWith its alternating 24-h light–dark cycles, changing day lengths with seasons, and weather conditions, has major effects on the physiology and behaviour of organisms from prokaryotes to mammals
The Need to Identify Current Gaps of Knowledge in Daylight ResearchOur rotating planet, with its alternating 24-h light–dark cycles, changing day lengths with seasons, and weather conditions, has major effects on the physiology and behaviour of organisms from prokaryotes to mammals
Does the fact that the spectral power distribution of daylight goes beyond the visible range make a difference for non-visual functions, when compared to electric light? For example, the infrared (IR) portion that is always present in daylight is non-existent in electric light

Summary

Introduction

With its alternating 24-h light–dark cycles, changing day lengths with seasons, and weather conditions, has major effects on the physiology and behaviour of organisms from prokaryotes to mammals. In other words, when work, social and environmental pressures disrupt the entrainment of circadian rhythms by light and non-photic zeitgebers (food, physical activity), the out-of-sync circadian physiology may predispose to altered stress levels and lifestyle changes (fatty diet, alcohol consumption, smoking, self-medication, lack of exercise, short sleep) Together, this will alter multiple brain and body circuits that underpin physical and mental health, representing a multifactorial basis for any form of pathophysiology [27,28]. Scheduled daylight and mixed daylight/electric light exposure conditions may optimise the effectiveness of certain medications/drugs and reduce side effects directly or indirectly via inducing robust circadian rhythmicity [29] The latter would enable timed coordination between the kinetics of a Clocks&Sleep 2020, 2 drug’s actions and the host’s responsiveness. These mixed/synergistic effects are missing for daylight and for mixed electric/daylight conditions

How Does Daylight Impact on Physiology and Behaviour Beyond Vision?

Gaps of Knowledge on Circadian Entrainment

How Does Daylight Influence Light Sensitivity Functions?

Is Daylight Exposure Effective as a ‘Treatment’?

Gaps of Knowledge Related to Individual Differences

Gaps of Knowledge in Daylight Research for Work Conditions

2.10. Gaps in Knowledge of Daylight Research Related to Environmental Factors

Gaps Group II

Gaps Group III

What Is Needed to Fill These Gaps and Achieve the Goals in Daylight Research?

Summary

Highlights