Drowning in Light

In 1996, Yale economist William D. Nordhaus calculated that the average citizen of Babylon would have had to work a total of 41 hours to buy enough lamp oil to equal a 75-watt light bulb burning for one hour. At the time of the American Revolution, a colonial would have been able to purchase the same amount of light, in the form of candles, for about five hour’s worth of work. And by 1992, the average American, using compact fluorescents, could earn the same amount of light in less than one second. That sounds like a great deal.

Except for one thing: We treat light like a drug whose price is spiraling toward zero. In the words of sleep expert Charles A. Czeisler of Harvard Medical School, “every time we turn on a light, we are inadvertently taking a drug that affects how we will sleep and how we will be awake the next day.”¹ Our daily metabolic cycles are not precisely 24 hours long, and this turns out to be a crucial evolutionary glitch in the mammalian circadian system. Circadian rhythms must be reset daily to keep us in behavioral synch with the earth’s rotation, so we will sleep when it is dark and wake when it is light. This process is called entrainment, and it is achieved by means of light exposure.  In the brain, a region of the hypothalamus called the suprachiasmatic nucleus receives input from the retina, causing specialized “24-hour” cells to oscillate in specific patterns. This affects how we eat, sleep, and work. And in most people, the circadian response is intensity-dependent, meaning the greater the light, the greater the effect on the human circadian system.