Daylight access in home offices improves circadian alignment, sleep, and emotional well-being

a blog by our science advisor Mariana Figueiro

Circadian rhythms are internal recurring cycles that find expression in a multitude of physiological, psychological, and behavioral processes across myriad animal and plant species. The master clock that generates these circadian rhythms would otherwise free-run at a natural period that is slightly longer than the solar day, and external environmental cues maintain synchrony—or entrainment—between the master clock and the time of day at our local position on Earth. The master clock’s primary environmental entrainment cue is the 24-hour pattern of light and dark incident on the retinae that is associated with Earth’s axial rotation.

The circadian system sends an alerting signal throughout the body during the day and sleep signal at night in all diurnal animal species, predominantly through the regulation of hormones. The cyclical rise in melatonin levels in the hours preceding bedtime, for example, triggers the onset of sleep. Conversely, the rise in cortisol levels in the hours before waking informs the body of the night–day transition from inactivity to activity. In an entrained individual, these two signals or processes work in a homeostatic relationship that balances daytime alertness with consolidated nighttime sleep. Misalignment of this relationship through inadequate or poorly timed circadian-effective light exposures is strongly associated with risks for various major diseases, declines in performance, and sleep and mood disorders. These risks demonstrate a clear need for applications that promote circadian health in an indoor built environment where lighting has typically been specified to benefit the human visual system rather than the circadian system.

This four-week, within-subjects, crossover design field study investigated the effectiveness of electrochromic (EC) glass windows on measures of circadian phase, sleep, vitality, and mental health in a residential environment. (EC glass changes from clear to tinted via the application of low-voltage electric current to a thin electrochromic coating embedded within its layers, reducing glare and thermal discomfort but not the penetration of circadian-effective light.) The study’s 20 participants were healthy office workers (median age of 33 years) who spent most of their time working at home due to restrictions associated with the COVID-19 pandemic. They experienced two experimental conditions (separated by a six-day washout period) in randomized order: (1) one eight-day intervention period with functionally standard (i.e., clear) windows with blinds partially drawn and (2) one eight-day intervention period with functioning EC glass windows. We hypothesized that compared to baseline measures, the EC glass condition would increase participants’ access to daylight and thereby lead to stronger alignment of rest-activity patterns with the day–night cycle and improved objective and subjective sleep and mental health outcomes.

We found that the study’s participants showed greater circadian alignment (as shown by the consistent dim light melatonin onset times), earlier and more regular sleep times, and improved vitality and mental health when living in an apartment fitted with the EC glass compared to traditional glass windows and blinds. In sum, the study confirmed that architectural designs effectively and comfortably incorporating daylight can yield tangible benefits for daytime office workers, even when working from home.

Short Summary: Circadian system disruption from inadequate or poorly timed light exposures is strongly associated with risks for various major diseases, declines in performance, and mood disorders. Our study found that electrochromic glass windows that reduce glare and thermal discomfort but still permit the penetration of circadian-effective light promoted circadian system alignment, earlier and more regular sleep times, and improved vitality and mental health.

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