Until 1879, sleep was governed by when the sun rose and set. Then some bright spark named Thomas Edison invented the lightbulb and everything changed. Suddenly, people could toil around the clock and stay up later when they weren’t working. A circadian rhythm based on the Earth’s natural light/dark cycle was disrupted for good. In the ensuing years, technology like the television, computers, tablets and phones have compounded the issue, flooding our eyes with round-the-clock artificial light if we can’t tame the urge to turn on and plug in.