Light has a profound effect on our sleep and waking. Humans are diurnal, which means we evolved to sleep at night. Despite what some people may claim about their personal habits, it is easier to sleep in the dark than in the light, thanks to something known as your circadian cycle.
The circadian cycle
External environmental and social cues set the phase for the circadian cycle. The most powerful cue is daylight. Sunrise, high noon, and sunset are milestones in nature’s clock – a clock that encompasses and influences plants, animals, and human society. The ganglion cells in the eyes’ retinas are connected directly to the suprachiasmatic nucleus (SCN), the “master clock” for the body.
Light is so important in establishing the circadian time that it is safe to say that it swamps all other signals. When people get jet lag, the “cure” is getting used to the local time, and the best way to accomplish that is by spending time outdoors in natural sunlight.
Light exposure can have profound effects on sleep. When the environment transitions from darkness to light, our brains tend to wake, whether we have had our regular sleep or not. It is harder to sleep in bright light than dim light. But exposure to bright light during the day also makes sleep feel deeper and more satisfying the next night.
Your circadian clock dictates much more than just your sleep patterns. It also impacts many of your internal functions from other body organs, including appetite, cell regeneration, and brain activity.
Circadian rhythm disorders
When the body’s internal biological comes out of synch with the external environment, it can develop into a circadian rhythm sleep disorder. The disorder may result from a temporary condition (such as jet lag), a job (as with shift work sleep disorder), or a biological condition such as blindness (which corresponds with free running circadian disorder).
People suffering from circadian rhythm disorders often use light therapy, melatonin, and medication to manage symptoms.
How light affects sleep
Exposure to light during the day keeps your energy levels up and improves your mood and cognitive performance.
People perform better on cognitive and physical tasks when it is light than when it is dark, independent of whether they feel sleepy. Just one hour a day of bright light has been shown to create these effects.
Evidence from brain imaging indicates that the waking brain is more active when the ambient light is bright. Even if we are not conscious of it and even if we find bright light distracting, we are more mentally nimble in bright light. Folk wisdom has long held that sunshine makes you happy, and indeed formal psychological testing has found positive effects from daylight.
Exposure to light during the day also helps you fall asleep at night, since your body recognizes the contrast with darkness in the evening and signals melatonin production to start.
The midday light exposure also increases release of melatonin at night. The circadian phase doesn’t shift, providing a biochemical basis for the claim that being outdoors during the day can help you fall asleep at night.
Night shift work and exposure to light at night has been linked to breast cancer, heart disease, obesity, and prostate cancer. The culprit may be that light reduces melatonin production – the hormone key to regulating your sleep-wake cycle.
Importance of darkness for sleep
Light signals to the body that it should be awake. On the other hand, darkness signals that it’s time to sleep. Often referred to as the sleep hormone, melatonin is also known as the darkness hormone, since its production coincides with darkness. Your brain begins releasing melatonin a few hours before you go to bed, reaching its peak in the middle of the night.
Lux is a way of measuring light, by factoring in both brightness and distance. For an idea of how strong artificial light is when compared to the natural nighttime light of the moon, take a look at the chart below.
Type of lighting
Illuminance (in lux)
32,000 to 130,000
Partly cloudy sunny day
Overcast day or shade
Office with windows
Dimly lit spaces
Take a look at the Lux of natural sunlight, compared to artificial light. Even an overcast day is more than ten times as strong as the lighting we find indoors.
On the other hand, comparing the Lux of indoor artificial light against that of a full moon demonstrates why artificial lighting interferes with our ability to fall asleep. Artificial light is hundreds of times more bright than darkness, which is why it keeps us up at night.
Artificial light and sleep
The retina in your eye is responsible for detecting the presence or absence of light. When it starts getting dark outside, the retina sends a message to the hypothalamus in your brain, signaling that it’s time to start producing melatonin, lowering your body temperature, and preparing your body for sleep. The reverse process happens in the morning when the your retina detects morning light and wakes your body up.
However, sunlight isn’t the only kind of light your eyes encounter. Thanks to artificial lighting, you can expose your retina to light no matter whether it is day or night, so your body gets confused about when it should be asleep and when it should be awake.
Artificial light can wreak havoc on your sleep, which is why you should never sleep with the lights on. In fact, sleeping with the lights on can decrease your melatonin levels by as much as 85 percent, according to a 2011 study.
All artificial light can impact your sleep, but our eyes are especially sensitive to blue light found in electronic devices (televisions, computers, cell phones, e-readers) and energy-efficient LED and fluorescent lightbulbs.
Blue light and sleep
Why is blue light so disruptive for sleep? It’s the same reason why it is so helpful for staying awake during the daytime. The blue wavelengths in natural and artificial light boost your attention and mood, helping you stay awake and alert during the day.
The problem arises in the evening, when the body needs to go to sleep. In the hours before your regular bedtime, your brain starts to release melatonin as part of its cycle, in accordance with your body temperature dropping. Exposure to blue light can delay the process, and preventing the proper amount of melatonin from being released.
The problem is not the blue light itself. Rather, the issue is with the levels of blue light contained in electronic devices, and the amount of exposure we have to them, especially at night.
Blue light in smartphones and tablets
If you are watching television, reading an ebook on your tablet, or checking social media on your phone, the blue light makes it difficult for you to fall asleep and experience a full night’s of sleep. As a result, you’ll end up missing out on REM sleep, which increases during the second half of the night.
Where do we encounter blue light?
E-book devices or e-readers
Fluorescent and LED lighting
Manufacturers have designed ingenious ways to reduce the blue light emitted by these devices. Many laptops, smartphones, and tablets now offer a bluelight shield option that filters out the blue light from the device. The result is a screen that relies more heavily on shorter red wavelengths, that are less energizing to the retina. For example, this feature is known as “night mode” on Apple devices.
Harvard University researchers found that blue light suppresses melatonin production and shifts circadian rhythm by a factor of two, when compared to green light of the same brightness.
Fortunately, there may be a solution: blue-blocking glasses. These are glasses with yellow lenses that filter out blue light.
In 2013, Toronto University researchers found that night shift workers who wore these glasses produced more melatonin that those who didn’t. These glasses can be beneficial for people who work in front of computer screens all day, or who like reading an e-book before bed. They can filter up to 60 percent of the blue light out.
A 2005 study measured the effect of bright light, dim light, and blue-blocking goggles on melatonin production in night shift workers. The chart below shows how the bright light almost entirely stopped their melatonin production. Wearing blue-blocking goggles, on the other hand, helped them produce nearly equivalent amounts of melatonin as those exposed to dim light.
Light therapy is used for people with circadian disorders, seasonal affective disorder, or jet lag. Although experts agree outdoor sunlight is more effective than light tables, the artificial light (if bright enough) can be useful when needed.
Light therapy is not the same as sitting by a regular lamp. Instead, these are bright white artificial light s(sometimes tinged with blue) that generate up to 10,000 lux, the goal being to mimic sunlight. You place these light tables or light boxes in the home, where normal lighting is less than 100 lux. Sitting by the light for 30 minutes can be enough to make an improved difference.
Light therapy can be used to help night owls wake up faster in the morning, and morning owls stay more awake later in the day.
Light therapy in combination with sleep restriction has been shown to be an effective way of raising mood in depressed people.
Low light is thought to be a cause of sleep disorders in people who are institutionalized and spend all day inside. It is suggested that bright light exposure can improve sleep quality in the population of Alzheimer’s disease communities.
People living in Antarctica in the winter often get delayed sleep phase syndrome or free-running disorder, as a result of severe lack of sunlight. Exposure to bright artificial light tinged with blue has been shown to help afflicted individuals get back on track.
Managing light exposure to improve quality of sleep
Even if you’re not suffering poor sleep to the point of needing light therapy, there are still some general guidelines you should follow to keep your circadian rhythms in sync.
1. Go outside during the day.
The contrast between a brightly lit part of the day and the dark night is important in telling the brain that it is time to sleep at night. Even brightly lit interiors such as office buildings and health clubs are dim compared to the outdoors. Brightly lit indoors might be 100 Lux, while outdoors on an overcast day is 10,000 Lux. You may not consciously register how different the light levels are because our eyes adjust to the environment, but even overcast days during the winter are bright compared to indoors.
2. Keep your bedroom dark.
You can use an eye mask or blackout curtains or drapes. Turn off any digital devices in the bedroom – such as computers, televisions, phones, or alarm clocks – or face them away from the bed when you go to sleep.
3. Limit your exposure to blue light at least 60 minutes before bedtime.
Instead of watching television before bed, try reading a book or meditating. Establishing a bedtime routine that you follow every night – and that doesn’t include television – will help ensure you don’t fall asleep with it on.
If your child is afraid of the dark and requires a night light, get a red or orange light. Red light is on the opposite side of the electromagnetic spectrum, and is the least disruptive to sleep.