Light is the most powerful external factor that influences our sleep. While most people intuitively know that it is easier to fall asleep when it is dark, the relationship between light and sleep is much deeper.

Light plays a central role in regulating the circadian rhythm, the body's internal clock that signals when we should be alert and when to rest.

Light also affects the production of melatonin, a key hormone that promotes sleep. Daily exposure to light, including the type of light we see as well as when and for how long we are exposed to it, has a critical impact on sleep.

In this article, we will analyze the complex links between light and sleep, which will allow you to shape your nighttime routine and your bedroom so that you enjoy high-quality sleep every night.

How does light affect sleep?

While human biology evolved with sleep following the daily patterns of sunlight and darkness, the advent of electricity changed this pattern, making the possibility of lighting available 24 hours a day.

From streetlights to cell phones, artificial light is a constant factor of modern life and has dramatic effects on sleep, affecting circadian rhythm, melatonin production, and sleep cycles.

Let's take a closer look at what exactly happens in each of these.

Circadian rhythms

The circadian rhythm is a 24-hour internal clock that coordinates a wide range of processes in the body, including sleep. This rhythm is controlled by a small part of the brain, known as the circadian pacemaker, which is strongly influenced by light exposure.

When light enters the eye, it is perceived by a special group of cells in the retina, which is transmitted to the brain and interpreted as information about the time of day. The brain then sends signals throughout the body to control organs and other systems according to the specific time of day.

When humans are exposed only to natural light, their circadian rhythm synchronizes exclusively with the sunrise and sunset, remaining awake during the day and in a sleeping state when it is dark outside.

In modern society, however, electricity creates a plethora of light sources that affect the brain's circadian pacemaker. Excessive or incorrectly timed exposure to artificial light can cause a person's circadian rhythm to become disoriented from the day-night schedule.

This can disrupt sleep and cause other health effects, such as a worsening metabolism, weight gain, cardiovascular problems, and perhaps even an increased risk of cancer . Circadian rhythms are also closely linked to mood, mental health, and seasonal affective disorder.

Melatonin

Melatonin is a hormone produced naturally by the body, and its production is inextricably linked to light. In response to darkness, the pineal gland in the brain starts producing melatonin, while exposure to light slows or stops this production.

Sleepiness increases with increased melatonin levels, which is one way this hormone facilitates sleep. Additionally, daily melatonin production cycles normalize the circadian rhythm, promoting a consistent sleep-wake schedule.

Some people with sleep problems may be given supplemental melatonin to help regulate their sleep schedule.

Sleep cycles

Sleep is not always the same. During a normal sleep period, a person goes through four to six sleep cycles, each lasting 70 to 120 minutes. These cycles consist of multiple stages of sleep, including both rapid eye movement (REM) sleep and non-REM sleep.

Exposure to light at night can disrupt the transitions between sleep cycles, reducing overall sleep quality. Too much light can cause repeated awakenings during the night, disrupting the sleep cycle and reducing the time a person spends in the deep stage of sleep, which is the most restorative for the body.

Circadian rhythm disturbances

Circadian rhythm disorders occur when a person's internal clock is not working properly or when it is not properly aligned with their environment. Many circadian rhythm disorders are linked to excessive light exposure.

Jet lag

Jet lag is a circadian rhythm disorder that occurs after long-distance air travel. When faced with the distinct day-night cycle of the time zone of the arrival city, the circadian rhythm can be disrupted. As a result, the person may have difficulty falling asleep, wake up earlier than they want, or experience excessive sleepiness during the day.

Dealing with jet lag usually means adjusting to the new time zone, such as by exposing yourself to daylight at certain times and avoiding light at certain times to re-adjust your circadian rhythm. This process can take several days and up to two weeks.

Shift work disorder

Shift workers typically have to sleep during the day, which puts them at high risk of having an out-of-aligned circadian rhythm and developing shift work disorder.

This circadian rhythm disruption can cause inadequate sleep, excessive sleepiness at inappropriate times, mood problems, and an increased risk of work-related accidents.

Other circadian rhythm disorders

Circadian rhythm disorders can occur when a person's internal clock shifts too far forward or backward, or when it doesn't follow a consistent 24-hour schedule. These problems are often linked to a person's daily behavior and exposure to light.

One treatment for many circadian rhythm disorders is phototherapy, which uses exposure to a high-powered light bulb at scheduled times to retrain the body's internal clock. The powerful light bulb is intended to mimic daylight and is often used in the morning to normalize circadian timing.

What types of lighting affect sleep?

Almost any type of light can affect sleep, but not all have the same impact. Daylight, which in direct sunlight has up to 10,000 lux, a unit of measurement of brightness , is much more intense than even bright office lighting, which rarely reaches around 500 lux. For this reason, daylight profoundly affects sleep and circadian synchronization.

There can also be significant differences between types of artificial light. Some types have greater brightness and intensity, and therefore even light that appears to be the same brightness as another may, in fact, have a different wavelength, changing how it is perceived by the eye and brain.

For example, blue light has a short wavelength and is emitted by many LED lights. Studies have found that it has a significantly greater effect on melatonin and circadian rhythm than light with a longer wavelength.

Many electronic devices, including cell phones, tablets, and laptops, emit blue light, and their extensive evening use can contribute to sleep disorders .

Why is it better to sleep in complete darkness?

As a general rule, it's best to sleep in as much darkness as possible. Complete darkness reduces potential distractions and sleep disturbances.

Sleeping with the light on interferes with sleep cycles and causes more fragmented sleep, and these disadvantages may be greatest in the few hours before waking.

Research shows that closing your eyes is not enough; our eyelids cannot block out light.

Effects on circadian rhythm can occur even with very low levels of indoor lighting and closed eyes. Even a small light indicating that an electronic device is off, such as a television, can affect the circadian rhythm.

Besides sleep quality, there are other reasons why studies show it's better to sleep in complete darkness:

• Eye strain: Even low levels of ambient light during sleep have been associated with eye strain , resulting in eye pain, fatigue, and discomfort, as well as greater difficulty focusing.
• Weight gain: Keeping lights on during sleep appears to affect the circadian regulation of metabolism, increasing the risk of weight gain, even if sleep itself is not disrupted. In one study over a five-year period, women who slept with lights or the TV on were significantly more likely to gain 10 pounds or more, even after controlling for factors related to diet and exercise habits.
• Cancer risk: An observational study found an association between people whose homes had high levels of artificial lighting at night and the risk of breast and prostate cancer. This study did not prove a causal relationship, and further research is needed to understand this association.

The various potential consequences of excessive artificial light in your bedroom demonstrate that light exposure can disrupt the circadian rhythm and its vital role in promoting many aspects of physical and mental health.

How to adjust the bedroom for better sleep?

A first step in setting up your bedroom to promote sleep is to make it as dark as possible. You can use, for example, blackout curtains that block out external light and create complete darkness.

When getting ready for bed, keep the lights low. Using a small, low-wattage bulb can help with the transition to sleep and complete darkness. Low brightness and a warm color temperature can help with relaxation and getting you in the right mood for sleep. One way to do this is to use candles or a lamp with warm orange and red colors.

Another step is to remove any electronic devices from your bedroom. The time you spend looking at a screen can stimulate your mind, delaying bedtime, and at the same time, the blue light it emits can disrupt circadian timing, melatonin production, and overall sleep.

If you can't completely remove electronic devices from your bedroom, reduce their brightness as much as possible, try not to use them before bed, and keep them on silent during the night.

Bright path to the bathroom

For many adults, especially older adults who are at risk of falls, it is important to be able to see where they are walking when going to the bathroom at night.

To combat this, first make sure the floor is clear of obstacles. Additionally, you can install motion-activated lights in the hallway or bathroom that provide low levels of warm red light, only when necessary and not all night long.

Change the settings on electronic devices

If you fall asleep with the TV on, it will continue to produce artificial light in your bedroom throughout the night.

The surest way to solve this problem is to remove the TV from your bedroom altogether. If that's not practical, check to see if the TV has a sleep timer that automatically turns off after a certain amount of time. If not, set a specific time each night when the lights go out and the TV turns off, and stick to that schedule to establish a consistent bedtime.

Frequently asked questions

What if someone is used to sleeping with the lights on?

Not everyone can sleep in the dark. Some people prefer to have light on while sleeping because it makes them feel more comfortable or because they are afraid of the dark. In these cases, lights should be set to the lowest setting. Timer lights can be beneficial so that even if you need light to fall asleep, most of your sleep is in the dark.

For severe phobias of the dark, consult a mental health professional, who may be able to develop a plan to minimize anxiety at bedtime.

Is there ever a time when sleeping with the lights on is better?

It's almost always best to sleep in the dark—however, you may want some light when you take a nap. Naps are usually more refreshing when they're shorter than 30 minutes, and having the lights on can prevent you from oversleeping and falling into a deeper sleep than you intended.

[Read: Why is success linked to naps? What does science say? ]

What other steps can improve sleep and circadian rhythm?

If you have long-term sleep problems that affect your energy, thinking, or mood, you should talk to your doctor, who can better determine the cause, including a possible circadian rhythm disorder.

Additionally, you can take steps to improve your sleep hygiene and reset your circadian rhythm . Avoiding excess caffeine, having a consistent sleep schedule, and getting regular exercise are examples of how your habits and routines can support healthy sleep.

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How do they work?

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