The Invention of the Light Bulb Fundamentally Changed Our Biology
Too much light at night, too little during the day: how our bodies are evolving to adapt to modern life
This story is a part of Elemental Light Week, a 5-day series on what light does for your body, brain, and well-being.
For centuries, humans slept in segments. They would go to bed around 9:00 p.m. or 10:00 p.m., sleep for three to four hours, and wake up after midnight for an hour or so. During that time they might pray, meditate, have sex, or even perform simple chores that didn’t require much illumination or skill. Then they would go back to sleep for another three to four hours, finally getting up around dawn. This strange sleep pattern wasn’t the result of insomnia, it was what’s known as biphasic sleep.
“Our sleep today — and by our I’m referring to people who live in North America and the Western world — is remarkably young. It’s an artificial product of modernity that arose around the late 19th, early 20th century,” says Roger Ekirch, a professor of history at Virginia Tech who was one of the first to publish on biphasic sleep. “That’s when the norm became consolidated or seamless sleep, to which we aspire — though not always successfully, of course — today. The transition was prolonged and erratic from segmented to consolidated sleep, but it generally occurred over the course of the 19th century with the first sleep that people took becoming elongated as the century unfolded.”
“If the light’s on, it’s day. If the light is off, it’s night, according to the brain.”
Artificial light was the cause of the transition — first through gas lighting and then the invention and widespread adoption of Thomas Edison’s incandescent light bulb. The technology made it possible (as we experience all too often today) for people to stay up working or socializing until 11:00 p.m. or midnight. As a result, people wouldn’t awaken from their first sleep until 3:00 a.m. or 4:00 a.m., and by that time, Ekirch says, they might as well get up and start working. As a result, sleep became more compressed and waking up in the middle of it was a…