But it was his study of the circadian rhythms of a blind college student that convinced him that artificial light was even more potent than he had believed. Czeisler was interested in the role that a subset of cells in the eye played in sleep. Even among people who have no rods and cones, the cells that translate light into vision, have a subset of light-sensitive cells in the eye. Those cells don’t allow them to see, but, he discovered, mean that even without the ability to discern whether a room is light or dark, a person’s sleep schedule can be shifted.

For years, Dr. Charles Czeisler has studied sleep. The chief of the division of sleep medicine at Brigham and Women’s Hospital knew that the invention of the light bulb had profoundly altered human life and biology. His work had shown that shifting sleep schedules could have dramatic effects on health.


Czeisler had studied this particular young man multiple times, including when he lived in a dormitory among people who could see and who used lights as normal sighted people do. As a freshman, the young man’s level of melatonin, a sleep-producing hormone, peaked at a typical time for normal people today: 4 a.m. But as an upperclassman, when he lived alone in a room with no artificial lights, his melatonin peaked at midnight. That suggested that exposure to artificial light is helping the rest of us suppress our melatonin levels.

“I began to think about it and recognize the light to which we’re exposed from dusk until we go to bed at night has actually had a profound effect and shifted us all as a society about four to five time zones westward -- as opposed to where we’re living,” Czeisler said in an interview.

In a perspective piece published Wednesday in the journal Nature, Czeisler argues that the sleep deficiencies that have become so pervasive among adults and children have been seriously exacerbated by progress. That first wave of progress occurred over the last half century. As technological advances allowed the cost of artificial light to decline, its use increased dramatically; Czeisler cites data that in the United Kingdom, a six-fold decrease in lighting costs over half a century corresponded with a four-fold increase in light use per person.


And now, another form of technological progress threatens our slumber: the ability to use LEDs that consume far less energy than incandescent bulbs.

The reason Czeisler is wary of LEDs, which are already in our laptops, televisions, and cellphones, is that they are projected to become even more abundant as governments retire the incandescent light bulb and shift toward more energy-efficient light. That matters because it isn’t just any artificial light that can reset our body clocks; the exact type of light makes a difference.

Shining light on that subset of light-sensitive cells, called ganglion cells, that triggered the blind college student’s hormonal sleep cycle to shift, inhibits the release of sleep-inducing melatonin. Those cells are particularly sensitive to blue light.

And blue LEDs happen to be widely used because it is relatively easy to make the light look white.

“The light is tremendously blue and rich, right at the wavelength that is most powerful at suppressing melatonin and resetting the circadian system,” Czeisler said. If more and more of our lights and gadgets bathe our eyeballs in a bluish glow, he fears, our epidemic of sleep deficiency may worsen.


Paradoxically, a shift toward LED’s may present one way to combat the problem, by giving people the ability to tune the color of light in their houses or on their screens -- so that only time-appropriate colors shine. That could decrease the likelihood that staring at a computer screen right before going to bed would impair sleep. But Czeisler sees both a big opportunity and the more probable alternative: a future in which light sources are manufactured without much regard for their biological consequences.

In his own house, Czeisler uses a software program called f.lux that tunes the LED displays of computers, shifting them to emit light frequencies that are appropriate for the time of day. He also keeps gadgets and televisions out of the bedroom and uses an old-fashioned incandescent light in the bathroom to avoid exposure to bright light at night.

Carolyn Y. Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.