How Daytime Light Exposure Affects Sleep, Mood-Related Outcomes

Spending more time outside during the daytime may significantly improve sleep-, mood-, and circadian-related outcomes, according to study findings published in the Journal of Affective Disorders.

Fundamental to general health, circadian rhythms are known to affect mood and sleep, with dim light at night and bright light during the day essential for both the robust amplitude of circadian rhythms and its appropriate alignment with the waking day, noted researchers.

In assessing modern human light exposure, increased use of electric lighting and light-emitting electronic devices at night have been indicated to blur the distinction between day and night, which may significantly alter the amplitude of circadian rhythms.

“A blunted circadian rhythm amplitude has been consistently reported in depressive disorders, which are characterized by symptoms including low mood, fatigue, and disturbances in sleep quality,” explained the study authors. “Insufficient exposure to daytime light could be a key factor contributing to poor mood and sleep outcomes in depressive disorders.”

With limited epidemiological data on the role of light in the regulation of circadian rhythms, sleep, and mood, researchers conducted the first large-scale cohort study of cross-sectional and longitudinal associations of free-living daytime light exposure with these outcomes.

Adult participant data were derived from the UK Biobank for 502,000 individuals (age range, 37-73 years; 54% women), recruited via National Health Service patient registers from 2006 to 2010.

“Participants provided detailed demographic, lifestyle, health, mood, and physical information via assessment and touch-screen questionnaire,” noted researchers. “Additionally, from 2016 to 2017, approximately 160,000 participants completed an extended online mental health questionnaire.”

Participant responses to daytime light exposure were limited to those observed in the typical day length in the UK during the summer (16 hours) and winter (8 hours), with responses of less than an hour a day recoded as 0.

Among the study cohort, a median of 2.5 daylight hours outdoors per day was observed (interquartile range [IQR], 1.5-3.5 hours).

After accounting for demographic, lifestyle, and employment covariates, each additional hour spent outdoors during the day was associated with several outcomes:

• Lower odds of lifetime major depressive disorder (odds ratio [OR], 0.96; 95% CI, 0.92-0.98)
• Lower odds of antidepressant usage (OR, 0.95; 95% CI, 0.92-0.98)
• Less frequent anhedonia (OR, 0.95; 95% CI, 0.93-0.96)
• Less frequent low mood (OR, 0.88; 95% CI, 0.87-0.89)
• Greater self-reported happiness (OR, 1.45; 95% CI, 1.41-1.48)
• Lower neuroticism (incident rate ratio, 0.96; 95% CI, 0.95-0.96)

Specific to sleep-related outcomes, greater time spent in outdoor light during the day was associated with greater ease of getting up (OR, 1.47, 95% CI 1.46–1.49), less frequent tiredness (OR, 0.81; 95% CI, 0.80–0.82), fewer insomnia symptoms (OR, 0.96; 95% CI, 0.94-0.97), and earlier chronotype (OR, 0.76; 95% CI, 0.75-0.77).

Findings of the longitudinal analysis of time spent in outdoor light at baseline with later mood-, sleep-, and circadian-related outcomes, as measured by auto-regressive cross-lagged models, supported cross-sectional findings, although generally with smaller effect sizes.

“These findings, while observational in nature, point toward future research that may improve our understanding of the role of light in the pathophysiology of mood and sleep disorders by directly measuring circadian physiology and daytime light exposure in a well-controlled longitudinal design,” concluded researchers.

Reference

Burns AC, Saxena R, Vetter C, Phillips AJK, Lane JM, Cain SW. Time spent in outdoor light is associated with mood, sleep, and circadian rhythm-related outcomes: a cross-sectional and longitudinal study in over 400,000 UK Biobank participants. J Affect Disord. Published online August 27, 2021. doi:10.1016/j.jad.2021.08.056