Daylight exposure, sleep and mental and physical health
Sleep is essential for our functioning and has an important restorative function. Sleep disturbances are prevalent, which increases with age to up to 40-70% in those at older ages (Miner & Kryger, 2017). Sleep disturbances are well known to be associated with increases of health problems, such as heart disease and dementia (Khan & Ouad, 2017; Shi et al., 2018), and poor mental, cognitive and physical health outcomes, including depression, anxiety, cognitive decline and physical frailty (Durmer & Dinges, 2005, Mander et al., 2017, Bao et al., 2017, Bubu et al., 2017, Dew et al., 1997, Freedland et al., 2012, Pourmotabbed et al., 2020). Sufficient sleep quality and quantity is therefore essential to maintain health ageing.
An important factor that predicts the quality of sleep is the alignment of the circadian rhythm and all its physiological consequences to the external day-night rhythm, which resets daily under the influence of the light-dark cycle (Golombek & Rosenstein, 2010). Disruption of the circadian rhythm is associated with increased risk of sleep disorders, psychiatric, neurodegenerative and several somatic disorders (Allada & Bass, 2021). Daylight exposure may therefore be an essential modifiable factor protecting individuals from poor sleep, and poor mental, cognitive and physical health outcomes.
While humans evolved under the circumstances of being outdoors most of the daytime, nowadays we spend most of the daytime under a roof in artificial light settings. Light intensity of artificial light does not reach that of natural daylight by far: daylight varies between 1,000 and 100,000 lux (very cloudy versus sunny), while artificial light varies between around 100 and 500 lux. Also, in contrast to artificial light, natural daylight is full spectrum (e.g. contains all colors) and dynamic (e.g. intensity and proportion of different colors change during the day). Spending most of our time in artificial light settings therefore likely affects our circadian rhythm, and associated health outcomes.
While studies in populations with a deviating circadian rhythm (e.g. shiftwork, jetlag, and a late chronotype) showed poor mental and physical health outcomes for these populations (Moreno et al., 2019), literature on the effects of daylight exposure in daily life is scarce and limited (Böhmer et al, 2021). Only recently, one study found lack of daylight exposure to be associated with increased risk of sleep problems and depressive symptoms (Burns et al., 2021). One other study found time spent outdoors to be associated with less depression in older adults (Harada et al., 2017). This study also found an indirect effect of time spent outdoors, mediated by physical activity, on cardiorespiratory fitness and lower extremity strength (Harada et al., 2017). Surprisingly, the literature on time spent outdoors in natural daylight is very limited and increasing time spent outdoors is not an intervention that is incorporated in clinical guidelines of mental or physical disorders (see https://richtlijnendatabase.nl). More evidence is therefore needed on the association of daily exposure to natural daylight with sleep, and mental, cognitive and physical health outcomes.
Age effects
The circadian rhythm becomes less stable with increasing age (Huang et al., 2002) and older persons more often wake-up during the night and proportions of their deep sleep stages and REM sleep decreases (Miner & Kryger, 2017). In addition, older persons are less sensitive to light exposure, and therefore may need more exposure to daylight in order to entrain the circadian rhythm and improve sleep and health outcomes than younger persons (van Someren et al., 2002). Therefore, daylight exposure might even be more important for older than for younger persons in order to maintain good sleep and mental, cognitive and physical health.
Potential confounding effects of somatic health status, disability, physical activity, and chronotype.
Disability due to poor somatic or mental health may disable the opportunity for individuals to go outside, as well as increase the risk of poor mental, cognitive and physical health outcomes. Therefore, disability may therefore confound the association between daylight exposure and mental, cognitive and physical health outcomes. Many persons are physically active outdoors, so physical activity may mediate the association of daylight exposure with mental, cognitive and health outcomes. Finally, chronotype may act as a potential confounder in the association. Chronotype refers to a person’s preference of waking up and falling asleep early (larks) or late (owls). About 15% of the population identify as larks, another 15% as owls with the remaining 70% as having an intermediate chronotype (Roenneberg et al., 2007; Refinetti, 2019). Owls are more prone to depression and anxiety and sleep longer than larks (Adan et al., 2012; Antypa et al., 2016; Keller et al., 2017), and due to their rhythm may be less exposed to daylight than larks. Therefore, chronotype may either confound the association, but may also moderate it (e.g. daylight exposure may have stronger effects on health for larks than for owls (Böhmer et al, 2021).
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