Wavelengths of light that shift a biological clock

Artificial lighting in the city at night encompasses the entire visible spectrum.

Animals experience internal daily rhythms which regulate many bodily functions. They are called “circadian rhythms,” which means they cycle at approximately 24-hour periods. Biological clocks synchronize these circadian rhythms with the cycle of night and day. Exposure of animals to light during a phase in their cycle when they would ordinarily experience darkness shifts their circadian rhythms.

A circadian rhythm regulates the timing of emergence of fruit flies from their pupae. A single 15-minute pulse of light interrupting darkness at a specified phase in their cycle resets their clocks and shifts their time of their emergence. William F. Zimmerman, my senior thesis advisor at Amherst College, and I found that wavelengths of light most effective for shifting the emergence time of fruit flies were in the blue spectral region. Effectiveness dropped sharply for wavelengths in the green spectral region. Wavelengths in the yellow, orange, and red spectral regions were ineffective, even at intensities 10,000 times greater than intensities that were effective in the blue region.

Our findings were published over 50 years ago. Many investigators have since confirmed the distinctive power of blue light to shift circadian rhythms in animals, including human beings.  

Light produced by lamps contains a mixture of wavelengths. Compared to yellow light, white light contains proportionately more energy in the blue spectral region. At night, restaurants typically avoid illuminating their dining areas with white light. By using yellow light instead, they theoretically spare their guests a circadian disturbance. For the same reason, the illuminated background of my Apple computer screen is set automatically to shift at night from white to yellow.

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View my published research on the biological clock of fruit flies.