Darkness is essential to sleep. The absence of light sends a critical signal to the body that it is time to rest. Light exposure at the wrong times alters the body's internal "sleep clock"—the biological mechanism that regulates sleep-wake cycles—in ways that interfere with both the quantity and quality of sleep. Melatonin, a hormone produced in the brain's pineal glad, is often known as the "sleep hormone" or the "darkness hormone." Melatonin influences sleep by sending a signal to the brain that it is time for rest. This signal helps initiate the body's physiological preparations for sleep—muscles begin to relax, feelings of drowsiness increase, body temperature drops. Melatonin levels naturally rise during the early evening as darkness falls and continue to climb throughout most of the night, before peaking at approximately 3 a.m. Levels of melatonin then fall during the early morning and remain low during much of the day. Evening light exposure inhibits the naturally timed rise of melatonin, which delays the onset of the body's transition to sleep and sleep itself.
For most of history, humans did not need to seek out darkness. The advent of electricity in the 20th century fundamentally changed our relationship to light and dark, and posed serious new challenges to sleep. Artificial light, inexpensive and ever-present, wreaks frequent havoc on sleep without many people even being aware of its detrimental effects. The widespread use of digital technology—and the light emitted from all those screens—has introduced another highly disruptive challenge to sleep.
Understanding how light is measured can help you manage your exposure to light more thoughtfully, and with an eye toward improving your sleep. There are a couple of measurements that are important in the world of light and dark: lumen and lux. Lumen is a measurement of light intensity or brightness, also known as radiance, at the source of the light itself. As light moves from its source, it disperses and its intensity changes. So when we're thinking about our exposure to light, it's not just the intensity of light itself that matters, it's also our distance from the light. That's where lux comes in. Lux takes lumen values and factors in the surface area over which light spreads. Lumen values can tell you how bright a light bulb is, but lux values can give an indication of how bright that light is in the space in which it—and you—reside. Lux measurements are also commonly referred to as "incident light."
Lux can be used to measure all types of light, both natural light and artificial light, and these values can vary tremendously depending on both the source of light, its power, and its proximity. Here's a little perspective on lux: On a sunny summer day, your environment might be in the range of 150,000 lux. Now imagine a cloudy winter day, when the sun is farther from you and obscured: a typical lux measurement on a day like that might be as low as 1,000. At night when darkness falls, lux values plummet. The moon generates values under a single lux.
A typically-lit home, with lamplight and overhead light, as well as light from outside, may have lux readings in the range of 300-500. The lux values of your environment at night will influence how easily your body prepares for sleep. In the evening hours, it's important to maintain low lighting, and allow your body to undergo its natural physiological move toward sleep. Appropriate lux for pre-bedtime activities in the evening, like reading, should be less than 180 lux. This level of brightness will allow you to be quietly active but won't impede your body's progress toward sleep. After light's out, your bedroom should be dark, with lux no higher than 5.
Managing your exposure to light in your home and in your bedroom is fundamental to creating a healthy sleep environment. With awareness, attention, and some simple planning, you can create a bedroom that guards against unwanted light at night, and protects the quality of your sleep until you are ready to wake. Curtains and shades on windows keep outside light from disturbing your sleep. Make sure window coverings are heavy enough to fully block light, and are well fitted to avoid slivers of streetlight or early morning sunlight from filtering in. Even brief exposure to light can interfere with sleep. Blackout curtains are designed to provide this kind of thorough protection against unwanted light.
If you need a source of light during the night—to make your way comfortably to the bathroom or to a child's bedroom—use a nightlight with a red bulb. Red is a long wavelength light that has been shown less disruptive to sleep than other light wavelengths. Put the nightlight in a hallway or another room, if possible. Having a small light in place will help you avoid having to flood your middle-of-night environment with unwanted, sleep-disrupting brightness.
The body needs time to prepare for sleep. A sleep routine that includes a gradually darkening environment can help. Dim the lights a full hour before bedtime to encourage your body to begin its physiological progression toward sleep. Use a dimmer switch on overhead lights to control their brightness, or install low-watt, dimmable bulbs in lamps. Avoid screen time the hour before bed: turn off the television, power down computers and tablets, and put your phone away for the night. The light from digital devices contains high concentrations of blue light, a wavelength of light that research has shown is especially detrimental to sleep.
An eye mask worn at night can help deepen darkness and protect against intrusive light. Choose a mask that is soft, comfortable, and flexible. Wearing an eye mask can take a little getting used to, but it is a highly effective tool for limiting your light exposure at night.
Being aware of light's effects on the body will lead you to pay more attention to the light that surrounds you, both day and night. Taking a little time to ensure a dark sleeping environment is one easy and important way to protect and improve your nightly rest.
Note: S+ is not a medical device. If you are seeking information on how to treat a sleep disorder, you should talk to your healthcare provider.