Circadian Rhythms

Lighting for Circadian Rhythms

A study of 23 people working in an open-plan office has been carried out by research scientists at BRE – with BRE Trust funding support – to translate experimental knowledge about the effects of lighting in the workplace into real-world good practice.

BRE Trust report summarising this project and providing advice on circadian lighting is freely available.

The research at a glance

Why this research?

Circadian rhythms are physical, mental and behavioral changes that follow a daily cycle. Found in most living things, they respond primarily to light and darkness in an organism’s environment. Sleeping at night and being awake during the day is an example of a light-related circadian rhythm.

Circadian rhythms control human alertness and sleep, and the release of hormones. Daytime exposure to light, especially blue light, helps synchronise the circadian clock, enabling us to feel alert during the day and sleepy at night. But many people work in poorly daylit spaces with relatively low levels of electric light, where it may be hard for their bodies to maintain their circadian rhythms.

Dynamic ‘circadian’ lighting is being marketed using dimmable, colour-tuning LEDs to give brighter, bluer light in the middle of the day, and dimmer light – with less blue – later in the day when it is time to relax. However, little or no research has been done on the best way to control this tuneable lighting under real-world conditions. Research was needed to help translate experimental knowledge into practice and investigate the effects of dynamic lighting and its timing on how people feel (i.e. their subjective assessments), and their activities and reported sleep.

Lighting conditions investigated

This research took the form of a BRE field study, with 23 participants working in an open plan office at the University of East Anglia. Four conditions were administered over several weeks during winter months:

Condition 1Old constant fluorescent lighting – i.e., the office’s existing lighting – from 19 February to 2 March 2018.

Condition 2New dynamic LED system – with variable LED lighting at a lower level – from 12-23 March 2018.

Condition 3New dynamic LED system – with variable LED lighting at a higher level – 12-23 November 2018.

Condition 4New dynamic LED system – set up to provide constant lighting – 3-14 December 2018.

Factors that were measured

Site measurements, lighting monitoring and computer modelling were combined with subjective and objective measures of performance – including questionnaires, regular pop-up questions and computer-based performance tests – along with the monitoring of light exposure and level of activity of participants using activity tracking watches.

The responses of the participants to questions and computer-based tests were assessed to identify links between key participant performance indicators – subjective alertness, reaction time and concentration – and the measurement and calculation results of circadian light metrics for each of the four lighting conditions.

Participant answers to general questionnaires following each lighting condition were also analysed and compared to assess the potential impacts of variable lighting. In addition, these results were correlated with the activity level data and the measurements of the site’s environmental conditions – temperature and relative humidity.

Research findings

Greater alertness

The average scores for subjective alertness were significantly better with the new dynamic LED system (Condition 2), than with the old constant fluorescent lighting (Condition 1). But comparisons of average subjective alertness scores with the LED systems set up to provide variable lighting (Condition 3), and constant lighting (Condition 4), revealed no statistically significant differences.

Extra light not a factor

Most participants felt more alert under the dynamic LED lighting in Condition 2 compared to the constant fluorescent lighting in Condition 1, but this also happened for the small number of people who received less light in Condition 2. The increase in alertness did not depend significantly on how much extra light people had with the LEDs. All participants received more light in Condition 3 compared to Condition 4, and the increase in light level was much more uniform across participants compared to the first conditions. However, the higher light levels in Condition 3 did not lead to higher scores, on average, for subjective alertness – only half of the participants felt more alert under the dynamic LED lighting (Condition 3).

Reaction time

Reaction time scores were significantly better with the new dynamic LED system (Condition 2) than with the old constant fluorescent lighting (Condition 1), both in the morning and across the entire day. However, there were no statistically significant correlations between these variations and the increases in circadian-weighted lighting metrics. There were no statistically significant differences in reaction time scores between Conditions 3 and 4, nor in concentration scores between the two conditions tested in each phase of the project.

Preference for dynamic lighting

In each phase, participants were asked whether they would prefer dynamic or constant lighting. On average, just over half of them preferred dynamic lighting for their office, typically brighter in the morning and following the variation of natural light outdoors throughout the day. Just under one third preferred the constant lighting.

More questions to answer

Overall, there is still considerable uncertainty about how much light is required for circadian entrainment – i.e. for a person’s circadian rhythm to align with the rhythms of light. People vary in their normal daily routines and in how much daylight they are exposed to. In addition, even in a space with ‘uniform’ electric lighting some people may receive significantly more light into their eyes than others, depending on which way they face.

More research is therefore still needed to understand better the potential impacts of lighting on circadian entrainment and wellbeing in real-life situations, and how to best quantify these in order to produce clear recommendations and guidelines for lighting than can support healthy circadian rhythms and wellbeing.

Daylight and solar shading guidance

One way of providing circadian lighting is the abundant provision of daylight in buildings. But this can also lead to issues of unwanted solar heat gain and glare unless carefully designed solar shading is provided.

An additional objective of this project, therefore, was to complete production of BRE guidance documents on solar shading. These were a Design manual for solar shading and two BRE Information Papers on retrofitting solar shading and control of solar shading – available from

Project outputs

The project findings are described in detail in various outputs, including the following publications: