Daylighting and Visual Comfort – Balancing compliance and wellbeing during shorter winter days

Why daylight matters in autumn and winter

As the clocks go back and daylight hours shorten, many people notice changes in mood, energy, and focus. Health professionals link this shift to seasonal affective disorder (SAD), which stems from reduced sunlight exposure. A lack of natural light can disrupt circadian rhythms and delay the body’s internal clock, affecting sleep patterns and hormone levels. People may feel tired, low, or find it harder to concentrate, which affects both wellbeing and performance. Getting outside during daylight hours, even for a short walk, is often recommended to help manage this.

This makes daylighting more than just a design decision. During autumn and winter, it becomes a genuine health consideration.

Understanding the technical side of daylight

Daylight calculations give designers a way to measure and manage natural light. These metrics show how much daylight will reach indoor spaces, how it will be distributed, and whether there’s a risk of glare. They’re often required at planning stage and feed into assessments like BREEAM visual comfort.

The key metrics include:

• Vertical Sky Component (VSC): Measures the amount of visible sky from a window. A VSC of 27% or more is typically seen as adequate, and any reduction below 80% of the previous value may be noticeable. VSC is often used to test the impact of new developments on existing homes.
• Daylight Factor / Illuminance: Expresses how much indoor light is available as a percentage of the simultaneous outdoor level. BRE 209 gives baseline values (e.g. 1.5% for living rooms), though more projects now use climate-based daylight modelling in line with BS EN 17037 and CIBSE guidance.
• Daylight Distribution (No Sky Line): Shows which areas of a room receive a view of the sky. A reduction below 80% of the original area is usually considered a significant loss.
• Annual Probable Sunlight Hours (APSH): Looks at how much direct sunlight a window could get in a year. BRE 209 sets 25% annual APSH (including 5% in winter) as a baseline. Only windows facing within 90 degrees of south are assessed. If APSH falls below the threshold and drops more than 20% from the existing level, the impact is likely to be felt.

While these aren’t statutory requirements, they are routinely used by local authorities to guide planning outcomes. They help strike a balance between daylight access, privacy, and the limits of a site. Many consultants also use 3D climate-based daylight modelling, which shows how light levels change over time and helps guide design.

Design and wellbeing – daylight, heat and performance

As summers become hotter, regulation now also aims to control unwanted solar gain. Approved Document O sets limits on glazing and requires adequate ventilation in new homes to prevent overheating. It includes a simplified route with ratios for glazing and ventilation, and a dynamic modelling route based on CIBSE TM59. The goal is to support health and welfare, though it doesn’t promise thermal comfort.

To design well-lit, comfortable spaces, teams need to assess daylight and overheating together. Options like external shading, solar control glazing or opening rooflights can reduce solar gain without sacrificing natural light.

There’s strong evidence that good daylighting improves health and performance. CIBSE’s guidance highlights that daylight helps create bright, comfortable places – not just for visual tasks but for how people feel. Cited studies show that workplace productivity improved by around 15% after moving to buildings with better daylight. Daylight during winter also helps boost mood and energy, supports sleep regulation, and improves cognitive function.

Daylight also affects energy use. Well-lit spaces rely less on artificial lighting. However, high daylight levels can raise cooling demands. That’s why it’s important to combine glazing with shading, ventilation, and thermal mass.

Plan early and think holistically

Daylighting needs to be considered from the start of a project. BRE 209 encourages testing at feasibility stage, supported by 3D models that inform early design choices like massing and window placement. This early input can help flag low VSC or APSH values and give time to adjust the layout, move windows or add rooflights.

Designers also need to link this work to overheating assessments under Part O. This might mean adjusting glazing areas, specifying solar control features or improving natural ventilation. Orientation matters too. South-facing elevations often need shading, while north-facing spaces may need larger glazing to meet daylight targets.

For projects using BREEAM, daylight work also feeds into HEA 01 visual comfort credits. Outputs must be clearly documented for planning. There are simple ways to improve light levels – using high-reflectance surfaces, selecting glass with appropriate transmittance and adding adjustable shading to manage glare.

In hospitals, schools, and other places where people spend long hours indoors, good daylight and access to views is especially important. For offices, new guidance such as melanopic EDI (Equivalent Daylight Illuminance) from CIE S 026/E:2018 supports the idea that bright, daylight-like lighting can improve alertness and support healthy sleep.

What enevo can support you with

As autumn and winter draw in, it’s clear how less daylight can affect both wellbeing and compliance. Daylighting calculations provide a practical base for design, helping ensure windows are placed to meet BRE and BS EN 17037 standards and that risks like glare, heat and energy use are managed from the start.

enevo’s building physics team works across all sectors to support compliance and create better spaces. We test daylighting, glare and overheating using industry-leading tools, provide clear outputs for planning, and offer design advice that links daylight to ventilation and thermal performance.

Whether you’re designing a new-build, adapting an existing site, or targeting BREEAM credits, we can help you find the right balance between technical requirements and real-world outcomes.

Contact us for an initial conversation to see how we can support your team with daylight modelling (and many other environmental testing and assessment services) to enhance comfort, meet your brief and keep people at the centre of your designs.