Circadian rhythms are the platform for all biology and are observed in several physiological, psychological and behavioral processes, in particular the sleep–wake patterns [1
]. In humans, endogenous circadian rhythms are generated and regulated by the body’s master biological clock, operating within the hypothalamic suprachiasmatic nuclei (SCN) [2
]. In the absence of any zeitgebers (i.e., external time cues), this master biological clock free-runs with an average period of approximately 24.2 h. The 24 h pattern of light and dark reaching the retina entrains (or synchronizes) the timing of the biological clock to the 24 h solar day.
The master clock promotes alertness during the day and sleepiness at night in all diurnal species, predominantly through the regulation of hormones. The rise of melatonin in the hours preceding sleep triggers sleep onset. Conversely, cortisol increases in the hours prior to waking and informs the body of the night–day transition (or between inactivity and activity). These endogenous signals generated by the master clock oppose the homeostatic sleep drive to promote daytime alertness and nighttime sleepiness [3
]. Specifically, stable and high daytime levels of alertness and consolidated nighttime sleep are maintained when the phase relationship between the internal circadian timing system and the sleep-wake cycle is aligned.
As the primary environmental cue for the body’s master biological clock, light has been shown to impact sleep, mood, performance, alertness, quality of life, and overall health in various populations [4
]. Optimizing light for the promotion of circadian alignment in architectural settings involves ensuring the proper levels, timing, duration, and spectra of light that occupants are exposed to throughout the day while minimizing their exposure during the evening, prior to bedtime. Daylight is an ideal light source for promoting circadian alignment due to its natural 24 h cycle, high light levels, and spectral power distribution which provides the proper circadian stimulus at all times of day.
Previous research monitoring 109 participants across seven consecutive days demonstrated that high levels of circadian-effective light during the entire day were associated with better alignment between light–dark and rest–activity patterns [6
], better sleep quality, and lower depression scores. In a 2014 study involving 49 office workers, Boubekri et al. [7
] reported superior sleep quality, increased sleep duration, and increased physical activity in participants working in windowed environments with more daylight access. In a recent survey involving 593 middle-aged adults, an increase in daily outdoor time and brighter indoor environments was associated with significantly reduced feelings of anxiety and depression and fewer sleep disturbances [8
]. A recent study across 12 European countries also found that daylighting conditions affect schoolchildren’s performance [9
Although these and other studies clearly demonstrate the benefits of daylight, modern lifestyles (largely spent indoors) afford fewer opportunities for receiving adequate daylight exposure. Enabling daylight effectively in indoor environments therefore presents a powerful opportunity for promoting improved nighttime sleep and better mood in humans, while also reducing energy use from electric lighting. While abundant daylight access is the design intention for most modern buildings, in practice windows are often covered by blinds to control for unwanted heat and glare, effectively limiting daylight access and impeding proper exposure to daytime circadian-effective light [10
]. This issue is compounded by behavioral tendencies, wherein an occupant is unlikely to change the position of the blinds, even on cloudy days, once they have been adjusted to block direct sunlight, tendencies that have been demonstrated in both office and residential settings [11
]. Electric lighting is a modest substitute for daylight, as it is unable to match the light levels of sunlight in commercial applications and has a fixed correlated color temperature, while daylight varies in light level and spectral power distribution throughout the day [12
One modern technology which facilitates daylight in indoor environments, while also addressing the challenges described above, is electrochromic (EC) glass. EC glass works by applying a low-voltage electric current to a thin electrochromic coating embedded within windows layers, which changes the window from clear to tinted. EC glass can also automatically tint in response to the presence and timing of direct solar radiation, effectively eliminating the need for occlusion by blinds or shades since it mitigates dynamic glare and heat conditions. EC glass also allows for more penetration of short-wavelength light, which is optimal for affecting the circadian system. Due to its contributions toward designing smarter, more sustainable, and healthier buildings, EC glass has become adopted at scale in a variety of markets including offices, healthcare facilities, residential buildings, and airports, with projects greater than one million square feet and totaling approximately 100 million square feet of adoption worldwide. This technology has recently demonstrated improvements to occupant sleep duration, cognitive performance, and environmental satisfaction while mitigating physical discomfort symptoms such as eyestrain and headaches [13
]. The current study is intended to build upon this research by focusing on the non-visual effects of indoor daylight levels as delivered by EC glass technology relative to the use of conventional windows and blinds. Our theoretical construct was that daylight, if enabled effectively in indoor environments, will promote the circadian alignment of the endogenous rhythms (e.g., melatonin rhythm characterized by consistency of dim-light melatonin onset) with the local day–night cycle, leading to improved nighttime sleep and daytime feelings of vitality.
The aim of this four-week, within-subjects, crossover design field study was to investigate the effectiveness of EC glass designed to reduce glare and increase circadian-effective light indoors on measures of circadian phase, sleep, vitality, and mental health in a residential environment. Due to the COVID-19 pandemic, participants spent the great majority of the study period in their own apartment units and experienced two experimental conditions 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. A six-day baseline period preceded the participants’ first intervention period, and a six-day washout period separated the two interventions. We hypothesized that the EC glass condition would increase participants’ daylight access and thereby lead to both stronger alignment of rest-activity patterns with the day–night cycle and improved objective and subjective sleep and mental health outcomes.
Over the course of this four-week, within-subjects, crossover design residential field study, all participants experienced two six-day baseline periods and two eight-day intervention periods: (1) functionally standard (i.e., clear) windows with blinds partially drawn and (2) functioning EC glass windows. It was hypothesized that the EC glass condition, designed to reduce glare and increase circadian-effective light indoors, would promote stronger alignment of endogenous rhythms (melatonin rhythm characterized by consistency in DLMO) and rest–activity patterns (characterized by phasor magnitude and sleep regularity index) with the external solar cycle, leading to improved objective and subjective sleep and mental health outcomes.
Participants exhibited a 15-min melatonin onset delay (p
= 0.03) over the course of the intervention period when their apartments had untinted glass and blinds (i.e., the blinds condition), but a maintained consistent melatonin onset over the course of the week when the EC glass’s tinting was activated (i.e., the EC glass condition). They also demonstrated improved circadian alignment (phasor magnitude, p
= 0.04), went to sleep 22 min earlier (p
= 0.05), and had higher sleep regularity (p
= 0.001) while in the EC glass condition. These impacts were likely driven by the improved daytime circadian-effective light levels imparted by the EC glass condition, supporting well-established mechanistic models demonstrating the benefits of short wavelength light characteristic of daylight on human circadian rhythm alignment and sleep quality [34
]). However, in the current study, CS levels were found to be relatively low regardless of condition, an observation that could be attributed to the chest-level pendant measurements which have been previously shown to underestimate light levels reaching the eyes [35
]. It is also important to note that the dynamic tuning of daylight toward shorter wavelengths by the EC glass enabled positive circadian and sleep effects of daylight at lower indoor light levels and shorter intervention length (8 days) than those reported previously [36
]. Regardless, the results are in line with previous field studies investigating the impact of access to circadian-effective lighting or to daylighting indoors on sleep, mood, and alertness [6
The observed DLMO and sleep delays in the Blinds condition over the course of just eight days demonstrate the sensitivity with which the human body responds to the external cues of light or daylight. These delays observed in the study, if accumulated, could lead to further curtailed total sleep given social and work lives that necessitates waking up at a fixed time. In fact, these data support the work by Roenneberg and Merrow, who showed that one needs to be exposed to at least 2 h of daylight per day to avoid experiencing social jet lag [39
]. In addition, the fact that longer rapid eye movement (REM) periods occur in the later part of the sleep cycle [40
] suggests that curtailing sleep in the fashion observed in the current study may lead to less REM sleep. REM sleep has been shown to be crucial for learning, cognitive and social processing, and higher-level thinking implicated in creative problem solving [41
]. Previous research has found that greater daylight exposure in an office setting increases sleep duration at night and, in turn, improves decision-making performance, with test scores on 1.5 h cognitive simulations increasing by 79% on average after five days of exposure [42
Providing the optimal light conditions indoors not only impacts circadian phase and sleep but also impacts daytime energy levels and mental health by promoting an acute alerting effect, wakefulness, and vitality [43
]. In the current study, while in the EC glass condition, participants exhibited higher positive affect (p
= 0.035) and demonstrated a distinct cycle of high vitality throughout the day, low energy at night, and high vitality again after waking the next morning, a cycle that remained relatively consistent from the start to end of the week. Meanwhile, in the blinds condition, they exhibited a delay in peak vitality, higher nighttime energy levels, and lower morning vitality at the end of the week compared to the start. These results are consistent with those from Figueiro and colleagues, showing that subjective vitality increased after exposure to circadian-effective light during the day [32
Implementing proper circadian-effective lighting indoors is particularly important given the downstream chronic health consequences of sustained circadian and sleep disruption. Circadian misalignment has been implicated in cardiovascular disease by increasing blood pressure and inflammatory mediators and reducing heart rate recovery [44
]; metabolic diseases such as diabetes, obesity, and insulin resistance [46
]; Alzheimer’s disease [46
]; and impaired cognitive performance [49
]. Furthermore, evidence from studies focused on jet lag, night-shift work, and seasonal affective disorder suggest strong associations between chronic circadian misalignment and increased precipitation or exacerbation of mood disorders such as depression, anxiety, and bipolar disorder [50
]. More recently, day-to-day regularity of the sleep schedule has received particular attention for its association with poor mood and depression, irrespective of total time spent asleep [53
]. Moreover, older adults with irregular sleep patterns have been found nearly twice as likely to develop cardiovascular disease and metabolic abnormalities [54
], and college students with irregular sleep were found to have poorer academic performance [19
]. Chronic circadian misalignment has also been implicated in cancers of the breast, prostate, and colon [56
]. In fact, the International Agency for Research on Cancer (IARC) recently categorized nighttime shift work as a probable human carcinogen [57
]. The mechanisms behind these health conditions have been extensively studied in model organisms bred with and without proper circadian function or exposed to circadian-disrupting cues [58
The weeklong interventions investigated in the current study limits the ability to extrapolate the results to longer exposures. However, previous research suggests that phase shifts may be additive in the continued absence of sufficient circadian-effective daytime light exposure [61
]. In addition, most of the study population was healthy and young, limiting the generalizability of these findings and implications to other populations. For example, those who suffer from sleep impairments or Alzheimer’s disease have been shown to benefit from exposure to daytime circadian-effective light [63
], and may benefit to an even greater degree than healthy young adults due to the amount of room for improvement. The study was also limited in that the participants were not given any instruction as to the use of electric lighting during the study (except during DLMO collection). However, this was a random factor that was common to both conditions and reflects the realistic nature of the experimental design. Lastly, the current study focused only on two experimental conditions, one with functionally standard glass and blinds and one with dynamically tinting EC glass; other window façade configurations were not studied.
The within-subjects, balanced crossover study design with washout periods conferred many strengths to the study. The balanced crossover design ensured that the results were not an artifact of temporal factors or learning effects, and the within-subjects design allowed each participant to serve as their own control. Furthermore, the study utilized objective measures (the Daysimeter, salivary melatonin, and actigraphy) to reproduce the mechanistic pathway from daylight exposures to hormone levels, sleep behaviors, and daytime vitality. The greatest strength of the study, however, was the face validity of the study setting and interventions. This field study was conducted in a real-world environment, with participants living in their own homes experiencing a realistic set of daylight exposures for one week at a time without intervening in their lifestyle or behaviors.