Natural daily light exposure linked to medication use, mood symptoms, and sleep-wake patterns

Light is the strongest environmental cue that synchronises circadian rhythms^1–3, and light also acutely influences mood and alertness^1,4. The effect of light on the circadian system depends on several factors, including the wavelength^5,6, intensity⁷, timing and duration of exposure^8–10, an individual’s prior light exposure history¹¹, and their individual circadian light sensitivity^12,13. Certain patient groups and psychiatric medications are associated with altered circadian light sensitivity^14–16. Those experiencing depression exhibit reduced sensitivity during depressive episodes¹⁵, and acute (single) doses of Selective Serotonin-Reuptake Inhibitors (SSRIs), a popular class of antidepressant medication, profoundly increase the sensitivity of the circadian system to light¹⁷. Although the chronic influence of SSRIs on circadian light sensitivity has not yet been studied, an increase in light sensitivity may influence circadian function and mood in individuals undergoing antidepressant treatment.

How an SSRI-related increase in circadian light sensitivity impacts circadian regulation would depend on an individual’s light environment across the day. Even in the absence of medication, light exposure patterns play a key role in circadian rhythms and mood outcomes. Light exposure patterns that align with the natural light-dark cycle, such as bright daytime light and dim light/darkness at night, strengthen circadian signals^18,19, assist in normalising circadian timing²⁰, and are associated with lower risk of depression²¹ and improved mood outcomes²². In contrast, modern light patterns, such as bright evening light and less daytime light blur the day-night distinction²³ and delay circadian phase²⁴, resulting in disrupted and blunted circadian rhythms, commonly observed in mood disorders^25,26. SSRIs may amplify the effects of light on mood and circadian rhythms. Healthy light patterns paired with an SSRI-induced increase in circadian light sensitivity may normalise the reduced circadian sensitivity to light seen in people experiencing depression⁵, possibly explaining the superior efficacy of combined SSRIs and bright light therapy^27–29. However, SSRIs may also exacerbate circadian disruption linked to misaligned light patterns, contributing to the reduced treatment efficacy³⁰ and higher depression symptoms in evening types compared to morning types³¹. Thus, everyday lighting environments may shape mood symptoms, particularly for those taking an SSRI.

Antidepressant efficacy is highly variable^32–34, and everyday light behaviour may be predictive of mood symptoms, representing a promising target for intervention. Chronotype, an individual’s preference for the timing of daily activities, is associated with variable treatment responses³⁰, and this may in part be driven by differences in habitual light exposure³⁵. Identifying whether particular light exposure patterns are unique to certain populations or represent broader trends, and whether they relate to mood and chronotype across individuals, is critical in recognising modifiable environmental factors that contribute to mood disturbances. Ultimately, this knowledge could guide the development of tailored lighting strategies that complement pharmacological treatments, improve symptom management, and address the global rise of depression^36,37.

In this study, we compared light exposure patterns between people taking SSRIs and controls, and examined how light exposure patterns relate to mood symptoms and chronotype (morning-evening preference). By clarifying these associations, we aim to inform future work on potential light-based strategies that could complement antidepressant treatment and support positive mood outcomes.

76 individuals (18-40 years, M = 25.03, SD = 4.24, 92.11% female) completed at least one week of field light-monitoring and questionnaires assessing mood (DASS-21) and chronotype (MEQ). The sample included 38 individuals regularly taking SSRIs and 38 matched unmedicated controls with no psychiatric history. Table 1 presents participant characteristics, including age, BMI, habitual sleep/wake timing, and self-reported depression, anxiety, and stress symptoms (DASS-21).

Across the sample, participants were exposed to an average of 678.38 melanopic EDI during the two hours following wake, 886.43 melanopic EDI during the day, and 35.64 melanopic EDI within the two hours before sleep onset. On average, participants spent 5.22 hours/day in light above 50 melanopic EDI and 2.05 hours/day in light above 250 melanopic EDI. Light Regularity Index averaged 80.09 at the median melanopic EDI threshold, and 84.75 at the 250 melanopic EDI threshold across the whole sample.

There were no significant group differences between those taking an SSRI and control participants in the light exposure metrics. Descriptive statistics for light metrics are shown in Table 2. Across all models, as expected, people taking SSRIs reported significantly higher levels of depression, anxiety, and stress symptoms compared to controls (all p < 0.001). Group remained a consistent and strong predictor in each model, independent of light exposure or sleep timing.

In this study, we examined how objective melanopic light exposure patterns related to mood symptoms and chronotype under free-living conditions in individuals taking SSRIs and matched controls. Our findings show that, although overall light exposure patterns did not differ between groups, greater exposure to bright morning light and spending more time in light above dim-moderate light levels (50 melanopic EDI) independently predict fewer depressive symptoms across the whole sample. Furthermore, morning-types exhibited greater light exposure in the morning and daytime, spent more time in light exceeding both low and bright thresholds (50 melanopic EDI and 250 melanopic EDI), and demonstrated differences in regularity of light exposure at both median and 250 melanopic EDI thresholds compared to evening-types. These findings indicate that light behaviour plays an important role in mood regulation, regardless of medication status, and that morning-evening preference relates to differences in daily light exposure patterns.

Brighter melanopic light in the first two hours after waking was associated with fewer depressive and stress symptoms across our sample. Bright morning light may influence mood through multiple pathways. Morning light exposure can advance circadian timing^38–40, counteracting the delaying impact of evening light that is often linked to low mood and depressive symptoms²⁴. Brighter mornings may also strengthen the amplitude of circadian rhythms⁴¹, which are typically blunted in depressive disorders^25,26. Light information is also received in brain areas related to mood regulation, such as the amygdala and habenula^4,42,43. As a result, light acutely improves mood and has been shown to impact self-perception (i.e., how an individual may think about themselves)⁴⁴. Therefore, greater levels of morning light are likely beneficial for mood through both circadian and non-circadian mechanisms of the non-visual effects of light. Notably, the average morning light levels in our sample were significantly below typical therapeutic thresholds used in clinical light therapy⁴⁵, indicating that even low-to-moderate levels of morning light may be beneficial for mood. This suggests potential for adjunctive naturalistic light interventions, such as spending time outside, which may be more accessible than clinical light therapy, to be beneficial in both patients and the community in general. Future randomized trials are needed to determine whether changing everyday lighting behaviours can effectively improve mood symptom severity in clinical and non-clinical populations.

We found that spending more time across the day in light above 50 melanopic EDI was associated with fewer depressive symptoms. This suggests that simply avoiding prolonged time in dim light environments may be an important, yet largely overlooked, factor in supporting mood. By contrast, the amount of time spent in light above 250 melanopic EDI, the recommended minimum melanopic EDI during the day⁴⁶, was not associated with mood symptoms. This may in part reflect the fact that modern lighting environments do not sufficiently reach these levels^47–50. Both groups spent an average of ~2 hours above 250 melanopic EDI, which is lower than durations previously associated with lower odds of depressive symptoms⁵¹, and therefore may not be sufficient to influence mood outcomes. Alternatively, the association may be influenced less by the duration of exposure above 250 melanopic EDI and more by whether individuals receive any exposure above this threshold. The distinction between the amount of time spent above the threshold and simply receiving any exposure above it is difficult to clarify without longitudinal data examining how changes in light exposure relate to mood over time. The lower threshold of 50 melanopic EDI may better capture variability between participants in terms of extended time spent in relative ‘darkness’, throughout the day. Together, our findings highlight the importance of daily light exposure above dim-moderate conditions. However, it remains unclear whether reduced light exposure across the day precedes depressive symptoms or results from mood-related changes in behaviour.

Regardless of whether changes in light behaviour precede or follow the onset of mood symptoms, spending more time in dimmer lighting conditions may contribute to mood disturbances. Low daytime light exposure has been associated with the onset of depressive episodes in seasonal affective disorder⁵², atypical depressive symptoms in non-clinical groups⁴⁷, and greater depression risk in population-based studies⁵³. Insufficient time spent in light may contribute to blunted circadian amplitude²⁵ and reduce activation of intrinsically photosensitive retinal ganglion cells (ipRGCs)^15,54,55, which mediate direct mood and alertness effects via projections to mood and alertness-regulating brain areas^4,42,43. These pathways provide a link between low light exposure and depressed mood. Prolonged time spent in dim environments may also reflect or reinforce sedentary or socially withdrawn lifestyles, further exacerbating symptoms. Thus, low light environments may both signal and sustain poor mood, underscoring the importance of promoting brighter daytime environments as a potential preventative strategy.

Prior studies have shown that less outdoor light exposure relates to antidepressant use⁵¹ and that greater amounts of evening light increase depression risk²¹. However, we did not observe any significant differences in light exposure patterns between individuals taking an SSRI and controls. Our results may be explained by clinical heterogeneity in our sample. Our sample included some people taking SSRIs reporting minimal to mild depressive symptoms, and some controls reporting moderate depressive symptoms according to the DASS-subscale scores. This overlap in symptom severity across groups may have reduced the contrast in light exposure patterns previously seen clinical and non-clinical populations⁵⁶, masking group-level differences. Importantly, serotonin agonists and SSRIs are known to alter circadian light sensitivity in nocturnal animals^57–59, diurnal animals⁶⁰, and humans¹⁷, suggesting that similar light environments could impact mood differently in medicated versus unmedicated individuals. SSRI-induced heightened circadian light sensitivity could lead to greater disruptions or benefits from daily light exposure patterns compared to controls, even when overall light exposure patterns appear overtly similar. For example, bright light therapy tends to be more effective when combined with SSRIs than when used alone²⁸, highlighting the interaction between medication and light environments. Longitudinal studies examining changes in light exposure and circadian light sensitivity after starting antidepressant treatment are essential for understanding how SSRIs influence the mood-enhancing or mood-disrupting effects of everyday lighting in the context of individual-level circadian light sensitivity and light exposure.

Both groups had evening melanopic EDI well above the recommended 10 melanopic EDI threshold in the two hours before sleep⁴⁶. This may reflect lifestyle factors, as a large proportion of the sample were university students (69.7%), particularly in the control group (81.6%), which may contribute to differing routines and exposure to evening light. For example, students often spend evenings studying or socialising, and are more likely to work part-time evening jobs, potentially increasing exposure to artificial light at night.

Evening light is known to disrupt sleep and circadian rhythms, and can predict psychiatric disorders^13,21,23,61. However, we did not observe associations between evening light and mood symptoms in this sample. This may be in part driven by high overall evening light levels, which exceeded the recommended evening light exposure levels, meaning we had relatively few participants with ‘ideal’ evening light exposure. Additionally, previous studies have shown a > 50-fold difference in response to evening light among young adults¹², and this variability is likely amplified in a sample including those taking medications like SSRIs¹⁷. The association between evening light and mood may depend on individual sensitivity to light, with the same evening light environment resulting in more adverse effects for people with greater sensitivity. This may be particularly relevant here, given our observed mean evening light exposure (35.64 melanopic EDI) is close to the mean light level required to suppress melatonin by 50% during the evening in young adults¹². This level of evening light exposure would therefore amplify the impact of inter-individual differences in circadian light sensitivity. Together, differences in non-visual light sensitivity and uniformly high exposure may have masked relationships between evening light and mood.

We observed that greater morningness was associated with increased morning and daytime light, greater time spent in light, and differences in light regularity depending on the threshold used. We found that evening types exhibited more regular exposure to light using the 250 melanopic EDI threshold, but less regular exposure when regularity was assessed using each individual’s median melanopic EDI threshold. Regularity at a fixed 250 melanopic EDI threshold may reflect evening types consistent but lower light levels across the waking day, compared to morning types. Morning types tend to wake and start their day earlier, coinciding with their peak physical and cognitive performance in the early part of their day⁶², which may encourage seeking more bright morning and daytime light⁶³. In contrast, evening types often wake later and reach peak performance closer to bedtime, leading to less morning light, lower overall light exposure, and greater irregularity in their median daily light patterns. Notably, all of our time-based light metrics (i.e., morning, daytime, and evening light) were calculated relative to an individual’s sleep-wake timing on a given day, rather than with static clock times (e.g., 0700–0900). As a result, the differences in light exposure between morning and evening types are not solely due to morning types waking earlier, and therefore receiving more light in the “morning”. People with greater eveningness were exposed to less light above 50 melanopic EDI, suggesting that chronotype may affect how much light people seek out or avoid. It remains unclear whether these light exposure differences are driven by intrinsic chronotype traits that influence the propensity to seek light, or whether altered light exposure patterns themselves contribute to shaping morning-eveningness. Indeed, evidence from camping studies shows that when individuals are exposed to natural light-dark cycles without artificial light, variability in sleep and circadian timing decreases⁶⁴, indicating that environmental light may play a role in producing differences in morning-evening preference. Differences in light regularity patterns between morningness and eveningness may contribute to the known greater variability in circadian timing, increased sleep disturbances, and higher rates of mood symptoms in evening types^31,65–68. Irregular light input to the circadian system, relative to an individual’s median light exposure, may underlie some of these adverse outcomes. Future longitudinal and experimental research is needed to clarify these causal relationships and determine whether modifying light exposure can improve circadian regularity and mood, especially in evening types.

It is important to note that our sample was predominantly women, which likely reflects broader trends with women being twice as likely to experience depression⁶⁹, and more likely to participate in mental health research. The sex imbalance may limit generalisability of our findings across sexes. Women tend to be more sensitive to bright light⁷⁰, have higher rates of depression⁶⁹, and respond differently to SSRI treatment⁷¹. It is possible that sex moderates the relationships between light exposure patterns and antidepressant treatment or mood outcomes more broadly. Further research should aim for a more balanced sex representation and directly examine the potential interacting effects of sex on associations between light exposure, antidepressant treatment, and mood.

This study provides novel insights into objectively measured naturalistic light exposure patterns in relation to medication status, mood symptoms, and chronotype. While we found no differences in overall light exposure between those taking an SSRI and controls, greater morning melanopic light exposure independently associated with fewer depressive and stress symptoms, and longer cumulative time spent above 50 melanopic EDI was associated with fewer depressive symptoms. Additionally, brighter, more regular, and longer time spent in moderate or bright light were associated with morningness. These findings underscore the potential value of incorporating targeted field-based approaches, such as increasing morning light exposure and increasing the amount of time in at least moderate light, into interventions aimed at supporting mood in both medicated and unmedicated populations.

Ethical approval was obtained from the Monash University Human Research Ethics Committee (Project IDs: 22905, 20590, 31958). Participants provided written informed consent and were reimbursed for their time. All procedures were conducted in accordance with the Declaration of Helsinki.

We thank our participants for their time and effort. This study was funded by a National Health and Medical Research Council Ideas Grant (NHMRC, 1183472), Australian Research Council Discovery Project (220102812), Australian Research Council Linkage Project (200201403), and supported by a philanthropic donation from Beacon Lighting. JES was supported by an NHMRC Investigator Fellowship (2025333). RJF, MH, and EJB are supported by an Australian Government Research Training Program (RTP) Scholarship. The funders played no role in the study design, data collection, analysis and interpretation of data, or the writing of this manuscript.

EMM and SWC contributed equally to this work. RJF, EMM, and SWC conceived and designed the study. RJF, EMM, ACL., MH, MTB., EJB, and APW collected the data, and RJF. JES, and AJKP analysed the data. All authors contributed to the preparation of the manuscript.

Raw data is available from the corresponding author upon request.

Data analysis code is available upon reasonable request.

AJKP, EMM, and SWC have received research funding from Delos and Versalux, and AJKP and SWC are co-founders and co-directors of Circadian Health Innovations PTY LTD. SWC and EMM have also received research funding from Beacon Lighting, and SWC has consulted for Dyson.

Sean W. Cain, Email: sean.cain@flinders.edu.au

Elise M. McGlashan, Email: elise.mcglashan@unimelb.edu.au