Why eyestrain is the outcome we can pin down
Eyestrain has a direct physical pathway: when daylight is blocked or glare is uncontrolled, occupants face either too little light or uncomfortable brightness, and both can strain the eyes. A review of daylight performance in offices identifies glare as a major issue that must be addressed alongside increasing daylight availability to maintain visual comfort [3]. A design study for Egyptian offices found that a bio-adaptive building skin reduced daylight discomfort glare by up to 31% while keeping indoor illuminance within the recommended 500–2000 lux range [4]. That study measured glare and light levels, not eyestrain itself, but it shows that the physical conditions linked to eyestrain can be improved through design.
The crossover study provides the most direct occupant-level evidence. Because the same people worked in both office conditions, differences in eyestrain reports are more likely to reflect the window treatment than differences between individuals [5]. The 48% reduction in reported eyestrain is a substantial and specific result, and it aligns with the physical logic that better daylight access with glare control should reduce visual strain.
Headaches, drowsiness, and performance: suggested but not nailed down
The same crossover study found that workers were 77% less likely to report feeling depressed and showed a significant benefit to positive affect when they had electrochromic windows [5]. These are mood outcomes, not headaches or drowsiness, and the study did not report separate measures for headaches or sleepiness. The improvement in emotional wellbeing is consistent with the idea that better daylight access supports occupants, but it does not establish an effect on headaches or drowsiness.
Performance is mentioned in the daylight literature as a potential benefit. A review study states that proper daylight availability can significantly help improve the performance of office workers [3]. However, that review discusses performance as part of a broader review of daylight factors and does not provide a measured effect size for performance in the abstract. A simulation study of non-visual daylight potential in a Danish office found that certain design combinations could reduce daylight access by up to three times, leading to a higher risk of circadian disruption, especially in winter and under overcast skies [2]. Circadian disruption could plausibly affect alertness and performance, but the study modeled circadian light exposure rather than measuring drowsiness or performance in occupants.
What the evidence actually covers
Not all studies of daylight in offices measure occupant outcomes. A thermal-daylight optimization study for tropical open-plan offices focused on building envelope parameters and energy savings, and did not assess eyestrain, headaches, drowsiness, or performance [1]. Its relevance is to how daylight provision is designed, not to which occupant outcomes change.
The Egyptian design study measured glare and illuminance rather than eyestrain or headaches [4]. The Danish simulation study measured circadian light potential rather than drowsiness or performance [2]. The review study discusses performance and visual comfort but is a review, not a new measurement [3]. Only the crossover study directly measured an occupant-reported outcome among the four in the question, and that outcome was eyestrain [5].
The practical takeaway for office design
If the goal is to improve a specific occupant outcome, eyestrain is the one with the clearest supporting evidence: better daylight access with glare control was associated with roughly half as many eyestrain reports in a controlled crossover comparison [5]. For headaches and drowsiness, the current evidence does not provide a direct measured effect. For performance, the evidence is suggestive but not quantified in the studies available here [3]. Design choices that increase daylight while controlling glare, such as dynamic windows or adaptive shading, address the physical conditions most directly linked to eyestrain [4][5].
Sources used in this answer
Optimising thermal-daylight performance in tropical open-plan offices: A coupled OTTV and daylight prediction approach
The study optimizes thermal-daylight performance in tropical open-plan offices using simulation and identifies shading coefficient, window-to-wall ratio, obstruction angle, and visible transmittance as key parameters, but it does not assess eyestrain, headaches, drowsiness, or performance.
Non-visual daylight potential in offices: a parametric study
A simulation of a Danish office found that certain design combinations can reduce daylight access by up to three times, increasing the risk of circadian disruption, especially in winter and under overcast skies, but it did not measure drowsiness or performance.
ASPECTS AND ISSUES OF DAYLIGHT PERFORMANCE IN OFFICES: A REVIEW STUDY
A review of daylight performance in offices states that proper daylight availability can help improve worker performance and that glare must be controlled to maintain visual comfort.
Daylight Performance-Based Design Model for Bio-adaptive Building Skin for Office Space of Administrative Buildings in Egypt
A bio-adaptive building skin design for Egyptian offices reduced daylight discomfort glare by up to 31% while maintaining illuminance within 500–2000 lux, but the study measured glare and light levels rather than eyestrain or other occupant outcomes.
Access to Daylight and Views Improves Physical and Emotional Wellbeing of Office Workers: A Crossover Study
In a crossover study of 30 office workers, working behind dynamic electrochromic windows was associated with a 48% lower likelihood of reporting eyestrain, a 77% lower likelihood of reporting feeling depressed, and a significant benefit to positive affect compared with standard windows partly covered by a fixed shade.