Lack of Exposure to Natural Light Disrupts Chronobiological Cycles

A new study shows that lack of exposure to natural light in the workspace is associated with physiological, sleep, and depressive symptoms.

Researchers at Universidade Federal do Rio Grande do Sul (UFRGS; Porto Alegre, Brazil) conducted a cross-sectional study to evaluate the effects of exposure or lack of exposure to natural light in a rest/activity rhythm on cortisol and melatonin levels, as well as on psychological variables in humans under natural conditions. The study participants were split into two groups according to their workspace, with 10 employees in the window (W+) group, and 10 in the without window (W-) group.

All participants were women and wore an actigraph to measure activity and ambient light exposure for seven days. Melatonin and cortisol were measured from saliva samples collected at 08:00, 16:00, and 22:00. The Montgomery-Asberg (MA) scale was used to measure depression symptoms and the Pittsburgh Sleep Quality Index questionnaire (PSQI) was used to evaluate the quality of sleep. Study participants also answered the Self-Reporting Questionnaire-20 (SRQ-20) to measure the presence of minor psychiatric disorders.

The results indicated that the two groups exhibited similar activities and light acrophases. In relation to light exposure, the midline estimating statistic of rhythm (MESOR) was significantly higher in the W+ group, who also presented the highest amplitude of light exposure. Melatonin levels differed at two different times of day, with the W+ group showing a lower melatonin level at 08:00, but a higher level at 22:00. Higher cortisol levels in the W- group correlated with minor psychiatric disorders and depressive symptoms, while lower melatonin levels correlated with depressive symptoms and poor quality of sleep. The study was published on November 26, 2014, in Chronobiology international.

“The diurnal light cycle has a crucial influence on all life on earth. Unfortunately, modern society has modified this life-governing cycle by stressing maximum production and by giving insufficient attention to the ecological balance and homeostasis of the human metabolism,” concluded lead author Francine Harb Correa, MD, and colleagues of the chronobiology laboratory. “Our study demonstrated that not only may light pollution affect human physiology but also lack of exposure to natural light.”

For most animals, the timing of sleep and wakefulness under natural conditions is in synchrony with the circadian control of the sleep cycle, and all other circadian-controlled rhythms. Humans, on the other hand, have the unique ability to cognitively override their internal biological clock and its rhythmic outputs. When the sleep-wake cycle is out of phase with the rhythms that are controlled by the circadian clock—e.g., during shift work or rapid travel across time zones—adverse effects may ensue.

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Universidade Federal do Rio Grande do Sul