Motivation and emotion/Book/2025/Lighting and mood

Imagine this ... Meet Sigurd, a resident of Tromsø, Norway, who experiences this polar night each year. As the daylight fades to almost nothing (see Figure 1), Sigurd notices his energy draining, his mood sinking, and his nights stretching longer with restless sleep. He feels sluggish and disconnected, struggling to find motivation in daily tasks. Throughout this time, the lack of natural light disrupts his internal biological clock, making it difficult to maintain a regular sleep schedule and regulate his emotions.

Light is a fundamental element of our environment, helping us to shape not only how we see the world, but also how we feel, think, and behave, both at conscious and unconscious levels. Beyond just enabling us to see, light can influence our biological processes, emotional regulation, and cognitive performance (Vandewalle et al., 2009; Wurtman, 1975).

Lighting conditions unsuited to their environment, whether in the home, workplace, school or in public spaces, can disrupt biological and psychological processes that drive mood regulation (Bedrosian & Nelson, 2017). For instance, research has demonstrated that insufficient daylight exposure can lead to depressive disorders, such as Seasonal Affective Disorder (SAD) (Partonen & Lönnqvist, 1998; Rosenthal, 1984), while excessive or poorly calibrated artificial lighting has been linked with heightened stress, reduced concentration, and visual discomfort (Begemann et al., 1997; Knez, 1995).

Given that many people living in industrialised societies spend the majority of their time indoors, the quality and characteristics of artificial lighting have become a determinant of psychological well-being and both long and short-term mental health. This chapter considers the complex interplay of environmental, biological and psychological factors that underpin the relationship between mood and lighting.

Mood is a sustained, affective state that shapes an individual’s perception, cognition, and behaviour over extended periods of time (Clark et al., 2018; Sekhon & Gupta, 2023). Unlike brief and intense emotions, such as fear or joy, moods are enduring and are less directly tied to specific stimuli (Beedie et al., 2005). Moods can vary along a spectrum ranging from positive states, such as happiness and contentment, to more negative states such as sadness and irritability. Research demonstrates moods are the result of both internal biological fluctuations such as hormonal and neurotransmitter changes and external environmental influences (Rautio et al., 2017; Wirz-Justice, 2006).

Mood regulation is influenced by multiple neurotransmitters and hormones, such as serotonin, dopamine and norepinephrine (Jiang et al., 2022; Liu et al., 2018), and imbalances in these systems are often linked to depression and anxiety disorders. Along with these major neuromodulators, several key brain structures are implicated in the shaping of moods, namely the amygdala, prefrontal cortex (PFC) and thalamus. These all aid in the integration of sensory information relevant to a person's mental and emotional state (Drevets, 2006).

Light plays an important role in regulating mood through its effects on both circadian rhythms and the body's hormonal systems. The body’s internal clock relies on external cues to align certain processes, including telling our body when to be alert.

The suprachiasmatic nucleus (SCN) in the hypothalamus (see Figure 2), receives light information and coordinates timing signals to peripheral clocks throughout the body which tell the body when to be alert or when to be at rest (Weaver, 1998). This light information is detected by intrinsically photosensitive retinal ganglion cells (ipRGCs), which are especially sensitive to blue light (Do & Yau, 2010). These cells send signals directly to the SCN through the retinohypothalamic tract, operating independently of vision and responding to overall brightness (see Figure 3). After receiving the information from the ipRGCs, the SCN then controls the release or suppression of melatonin and cortisol, which modulates the body's sleep-wake cycle and alertness in accordance with the amount of available light (Castaño et al., 2019). As it gets darker, melatonin levels rise to encourage sleep.

As previously mentioned, ipRGCs transmit light information without the need for visual processing. Specifically, mood regulation relies on the perihabenular nucleus (pHb) in the thalamus, and is necessary for light to induce mood changes in behaviour. The pHb is intergrated with several central mood regulatory systems, such as the medial prefrontal cortext and the nucleus accumbens, which are both regions strongly associated with emotional regulation, decision making and reward processing (Mure, 2021). The discovery of the pHb's role challenges several earlier models that place the SCN at the centre of all light-related effects on mood. Rather, it supports the notion that mood regulation is not soley regulated by circadian rhythms.

The impact of light on mood varies heavily depending on its source, with natural and artificial light each affecting the body in different ways. Sunlight is unique in its both its spectral range and dynamic nature, where it shifts in intensity and colour temprature throughout the day. These changes act as environmental signals to the brain for the regulation of circadian rhythms, mood, and cognition.

As light is the primary cue for synchronising circadian rhythms, regular exposure to daylight helps to stabilise the internal body clock, whereas irregular or poorly timed artificial lighting (such as the prolonged use of screens at night) suppresses melatonin and disrupts these circadian rhythms (Green et al., 2017; Tähkämö et al., 2018). Morning daylight exposure is linked to improved mood and more stable sleep-wake patterns, and has proved a key treatment component for seasonal disorders (Figueiro et al., 2017).

Natural light has a broad spectral range, including ultraviolet radiation, and temporal variations, which can have biological effects not accounted for in current standards for artificial lights. Higher illuminance levels are associated with improved alertness and cognitive performance, and exposure to bright natural light during the day can increase positive mood and reduce drowsiness. (Campbell & Dawson, 1990; Smolders et al., 2012).

The effects of natural light are not just limited to their biological benifits. Many studies show environments that are flooded with daylight (see Figure 4) are typically appraised as more calming and pleasant as opposed to artificially lit spaces (Gou, Lau & Qian, 2013; Küller et al., 2006). This is partially supported by the Attention Restoration Theory, which suggests that natural environments can support recovery from mental fatigue through soft fascination (a state of involuntary attention), however, recent systematic reviews have raised concerns regarding the generalisability of this theory (Ohly et al., 2016).

From a broader perspective, it is widely suggested that humans have an innate preference for natural stimuli, such as sunlight due to an innate attraction between biological systems (Gaekwad et al., 2022). Architectual features, such as window size and room orientation, play a role in determining how much natural daylight people recieve on any given day, and variations in this features have been shown to have measurable psychological effects on mood and emotional regulation (Küller & Lindsten, 1992).

Imagine this ... Despite it being a school night, Sarah decides to finish the new season of the show she has been waiting for. After a disappointingly anti-climactic finale, she looks up to find the clock is reading well into the early hours of the morning. Resigned to her fate, she sets her alarm to go off in a few hours, and lays back down. Despite her obvious signs of fatigue, the short-wavelength blue light emitted from the screen has confused her brain, and she will still be awake and unable to sleep when the morning sun begins to glare through her bedroom curtains ... (see Figure 5)

Artificial lighting, such as that from electronic screens, LEDs, fluorescents, halogens, neon lighting, and HID lights, are often inescapable in urban environments. Unlike natural light, artificial light is often static in both its spectrum and timing, which can serve to either support or interfere with an individuals' biological rhythms. Artificial light at night has been increasingly studied in relation to circadian misalignment and subsequent mental health risks, with evidence showing that even dim exposure to artificial light during sleep can impair mood regulation and dysregulate stress hormone timings (Cho et al., 2015). This is supported by Wittmann et al. (2006), who argued that since artificial light allows individuals to work against natural circadian rhythms and stay up during later hours of the night there is potentially a discrepancy between both biological and social clocks, creating a kind of "social jetlag".

Exposure to artificial light at night (ALAN) is widespread amongst 21st century society, highly present in many urban environments such as cities, hospitals and offices. Prolonged exposure to irregular light cycles, often experienced by night workers or patients in harsh, brightly lit wards (see Figure 6), has been strongly linked to the development of mood disorders and deficits in function (Bedrosian & Nelson, 2017; Bedrosian & Nelson, 2013).

At the same time, there is evidence to suggest there can be positive applications of ALAN. Blue light installations have started to become popular in urban spaces such as train stations in an effort to target suicide attempts, with an observational study in Japan reporting a 84% decrease in suicide upon successful installation (Matsubayashi et al., 2013). These lights are thought to reduce impulsivity and promote calmness (Liu et al., 2025). However, recent replication attempts have demonstrated limited success, suggesting a need for future research into the effectiveness of such light-based interventions (Erlangsen et al., 2023).

The potential of targetted artificial lighting has also been investigated within the context of clinical environments. Optimised hospital lighting can reduce stress and support patient recovery (Dalke et al., 2006), and blue-enriched light can increase alertness in night-shift workers (Campbell & Dawson, 1990; Chellappa et al., 2011; Smolders et al., 2012). However, findings remain uncertain, with studies on dynamic lighting systems designed to mimic natural light patterns showing results in increasing circadian health (Zhang et al., 2020), whilst other studies reported no significant effects on staff wellbeing (Simons et al., 2018).

The widespread global use of smartphones (and other similar electronic devices) has made ALAN a constant reality for much of the population. The short-wavelength blue light emitted from these devices has been shown to be particularly good at suppressing melatonin, impacting natural circadian rhythms and sleep patterns (Chang et al., 2015; Sinha et al., 2022). In a four week intervention study by He et al. (2020), participants who avoided their mobile phones for 30 minutes before sleep reported increased sleep quality and reduced pre-sleep arousal, further supporting the limitation of illuminated screen usage corresponding with increased positive affect.

Table 1.

Summary of light sources

Controlled exposure to specific lighting conditions has been applied to improve both general wellbeing and in the treatment of specific mood and sleep disorders, applying the biological effects of light on circadian rhythms, hormone release and alertness.

One of the most established applications is bright light therapy, which involves daily exposure to intense artificial light (around 10,000 lux) for up to 20 to 30 minutes a day (Kogan & Guilford, 1998). Bright light therapy has been shown to be particularly effective in the treatment of Seasonal Affective Disorder (SAD), where it helps decrease depressive symptoms brought on by the reduced daylight in winter months (Pail et al., 2011). Research also suggests it may have benefits outside the treatment of SAD, with several studies indicating improved sleep, energy and overall mood (Pail et al., 2011; Wirz-Justice, 2006).

New research is currently exploring the potential benefits of personalised or adaptive light systems in workplaces and schools, which allow for adjustments in intensity and colour temperature in an effort to support alertness, focus and mood throughout the day (Barkmann et al., 2012). Light therapy has also shown promise in cognitive improvement for those with ADHD, and as a potential alternative to medication for antepartum depression (Terman, 2007).

Light is more than simply a means of vision; it is an important regulator of human biology, cognition, and emotion. While light helps to shape and contribute to the determination of mood, alertness and long-term mental health through its influence on circadian rhythms, hormonal systems and neural processes, its impact varies depending on its source. Bright natural light helps to stabilise sleep-wake cycles and boost happiness through the secretion of serotonin. In contrast, the impacts of artificial light are highly context dependent, with inappropriately timed usage and prolonged exposure at night disrupting melatonin release, which can lead to negative mood states and poorer emotional regulation. Clinical applications, such as bright light therapy, help showcase the therapeutic potential of harnessing light exposure, and innovations into adaptive lighting suggest promising avenues for the enhancement of psychological health in everyday urban environments. Further research into the interaction between light and mood may help inform not only medical interventions, but the design of homes, workplaces, and public spaces that can better support societal wellbeing.

• Circadian rhythm (Wikipedia)
• Lighting (Wikipedia)
• Melatonin and circadian motivation (Book chapter, 2025)
• Mood (Wikipedia)
• Omega-3 fatty acids and mood (Book chapter, 2020)

• Blue light has a dark side (Harvard Health Publishing)
• How does lighting affect mental health in the workplace? (Forbes)
• How smartphone light affects your brain and body (Business Insider)
• How does the human eye work? (BBC)