If you’re reading this at night, beware: You may be affecting your body’s internal clock. Humans, like other animals, rely on light cues to set their body's daily cycle of activity, or circadian rhythm. Now a new study shows that some wavelengths of light, such as those from computer screens, have an unexpectedly strong influence on these rhythms, keeping us awake, for example, when we should be sleeping.
In mammals, a well-tuned system of light-sensing cells regulates the area of the brain that controls circadian rhythms, including those governing alertness and hunger. In diurnal animals, for example, light suppresses the production of the hormone melatonin, which is released during the night and promotes sleep. Researchers once thought that the eye’s rod and cone cells, which allow us to see, were responsible for detecting these light cues. But in some blind mice and blind people, circadian rhythms respond normally to changes in light exposure.
Scientists now suspect that neurons in the retina that contain melanopsin, a pigment that is sensitive to short-wavelength blue light, drive circadian signals; these cells are still functional in blind people whose body clocks respond to light and dark signals. As a result, some researchers have recommended blue-light therapy for seasonal affective disorder, a type of depression triggered by winter’s short days. Others have developed blue light-blocking goggles to help insomniacs sleep better.
But a new study of the specific effects of blue and green light suggests that the real story is more complicated. Neuroscientist Steven Lockley of Brigham and Women’s Hospital in Boston and his colleagues studied how exposure to different levels of light affected the sleep of healthy human volunteers, too. The researchers expected that blue light would suppress melatonin production and delay the subjects’ body clocks, making them go to sleep later. Green light, a wavelength that strongly activates the eye’s cone cells but does not activate melanopsin, would not alter the subject’s circadian rhythms, they also predicted. Blue light had the expected effects, strongly suppressing melatonin levels and delaying sleepiness. But unexpectedly in the first few minutes of light exposure, and at low light levels, the green light also suppressed melatonin levels and delayed the subject’s body clocks, Lockley and his colleagues report today in Science Translational Medicine. The observations suggest that cone cells do help to set the body’s clock after all.
That means that therapies that focus on blue light alone “may not be as effective as we thought,” Lockley says. “We need to think about the entire spectrum of light” when designing light therapy or lighting designs to boost alertness or induce sleepiness. The study authors also say that evening exposure to dim light sources, such as dimmable lamps or computer screens, could delay melatonin production, making it harder to fall asleep and to wake up feeling alert. Lockley says researchers had assumed that non-blue light had little effect on wakefulness, “but it’s not as safe as we thought” for people who are trying to avoid insomnia.
The study is “the most extensive assessment of the effects of blue and green light on human circadian physiology to date,” says Derk-Jan Dijk, a sleep physiologist at the Surrey Sleep Research Centre in the United Kingdom. He suggests that the results may help researchers design optimal light mixtures, not only for helping treat depression or insomnia but also for improving alertness during the day.