Morning blue light and afternoon red light best for office workers

Researchers from the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute and the U.S. General Services Administration (GSA) have recently published the latest in a series of studies exploring how light impacts alertness during the day and sleep quality at night in daytime office workers.

The study field-tested a novel luminaire developed by the LRC to promote circadian entrainment and alertness in the office environment. The luminaires provided varying levels and types of light during the day:

  • morning – saturated blue light delivering a circadian stimulus (CS) of 0.4
  • midday – polychromatic white light delivering a CS of 0.3
  • afternoon – saturated red light delivering a CS close to zero.

Nineteen participants from three U.S. Department of State office buildings in Washington, D.C., completed the three-week study. Objective and subjective measures of rest–activity, sleep, vitality, and alertness were used to evaluate the lighting interventions.

The results show that participants exhibited more consolidated rest–activity patterns, indicating better circadian entrainment. The morning blue light appears to have advanced participants’ circadian phase, causing participants to wake up earlier in the morning. The afternoon red light elicited an acute alerting response close to the post-lunch dip (around 3pm), reducing subjective sleepiness and increasing subjective vitality and energy.

These field results are the first to demonstrate that red light in combination with ambient white light provides an effective alerting stimulus, and support the inference that light exposures, when properly applied, can promote circadian entrainment and increase alertness.

The research paper, “Light, entrainment and alertness: A case study in offices” was published earlier in the journal Lighting Research & Technology. Authors include Mariana Figueiro, Mark Rea, Levent Sahin, and Charles Roohan from the LRC.

Previous LRC studies measured light levels for 109 participants at five federal office buildings designed to maximise daylight availability indoors. Dr Figueiro and her team found that even in open offices with many, large windows, office workers were not receiving enough light to stimulate their circadian system during the day, due to factors such as season, cloud cover, desk orientation, and window shade position.

In response to these findings, the research team theorised that supplemental electric lighting could be used to ensure that office workers receive enough light during the day, and installed circadian-effective lighting for 68 participants at four additional sites.

The study results showed that office workers felt much less sleepy with the use of supplemental electric lighting. They also reported feeling significantly more vital, energetic, and alert compared to baseline.

“The present findings show that a tailored lighting intervention can help entrain building occupants and can increase alertness during working hours. The ‘non-visual layer of light’ solution utilised in the present study is practical and inexpensive to implement, while helping to reinforce the bridge between laboratory results and field applications,” said Dr Figueiro.