A field study was conducted at two U.S. federal government office sites and two U.S. embassies to demonstrate whether circadian-effective lighting (providing circadian stimulus (CS) values of CS ≥ 0.3) could be installed in office buildings, and to determine whether this lighting intervention would reduce sleepiness and increase alertness, vitality and energy in office workers while at work. Desktop and/or overhead luminaires provided circadian-effective lighting at participants’ eyes during a two-day intervention. A pendant-mounted Daysimeter device was used to measure participant-specific CS values during the baseline and the intervention days. Participants also completed questionnaires inquiring about sleep habits, stress and subjective feelings of vitality and energy. The Daysimeter data showed that participants were exposed to significantly higher amounts of circadian-effective light while at work during the two intervention days compared to the baseline day. Self-reported sleepiness scores were significantly reduced during the intervention days compared to the baseline day. As hypothesised, participants also reported feeling significantly more vital, energetic and alert on the intervention days compared to the baseline day. The present results from four independent office environments demonstrate that lighting systems delivering a CS ≥ 0.3 can reduce sleepiness and increase vitality and alertness in office workers.