Ontime Health

Effects of Light on Sleep and Circadian Rhythm

improving sleep quality

Light can have a profound impact on our sleep. Exposure to light can stimulate wakefulness and alertness, whereas darkness can lead to sleepiness. This is because light acts as a signal for your circadian rhythm, the body’s 24-hour biological clock. 

By understanding how light can influence our body’s biological rhythms, you can control your light exposure to ensure optimal sleep quality and overall health. 

How does light affect sleep?

Light has a direct effect upon our feelings or alertness or sleepiness. Exposure to light makes us feel awake, alert and increases our ability to concentrate, whereas darkness can promote drowsiness and sleepiness. 

When we are exposed to natural light from the sun, our body’s internal clock, known as circadian rhythm, becomes aligned with daylight hours. Circadian rhythms are the body’s 24-hour natural clocks that play an important role in the timing and quality of sleep. If you have ever experienced jet lag, you will know how powerful these rhythms can be. 

Circadian rhythms control many mental and physical processes, such as: 

  • Body temperature
  • Hormone levels
  • Sleep-wake cycle
  • Alertness and drowsiness 

How does light affect circadian rhythm?

When light hits your eyes, it is detected by specialized cells in the retina that carry a signal to a part of the brain called the suprachiasmatic nucleus (SCN). The SCN uses this information to tell the time of day, and this information is transmitted to various organs of the body.

The absence of light sends a signal to the body that it’s time to rest through the release of melatonin, the ‘sleep hormone’, from the pineal gland in the brain. The release of melatonin signals the brain and body that it is time to sleep by inducing physiological changes including muscle relaxation and a decrease in body temperature. 

Melatonin levels rise during the evening and reach a peak around 3 a.m., before decreasing in the morning and remaining low during the day. However, nighttime light exposure reduces melatonin suppression, making it harder to fall asleep. In this way, light is an important signal for circadian rhythm that controls the biological and mental changes that occur over a 24-hour period. 

Other signals for circadian rhythm

Besides light, there are several additional signals that the brain uses to control circadian timing, known as ‘zeitgebers’. These environmental cues are used by the body to determine the time of day, and include:

  • Food intake
  • Sleep timing
  • Exercise timing 

How is light measured?

Light is often measured in a unit called lux, which is often referred to as ‘incident light’. This unit of measurement takes into account the brightness of the light source, and your distance from it. 

Lux can be used to measure both natural and artificial light. On a bright day in summer, the outside environment may be 150,000 lux. On a cloudy day, this can fall to 1000 lux. At night, the outside environment drops to below a single lux. 

On the other hand, a well-lit house may register up to 500 lux. In the evening, it is important to dim lights to less than 200 lux to help your body prepare for bed. When you turn off the lights before sleep, you should ensure your bedroom has no more than 5 lux. 

Sleep cycles and light exposure

Sleep is not a uniform process. In a normal night of sleep, you will go through between four to six sleep cycles, each lasting 90-120 minutes. Each cycle is made up of 4 different stages: two stages of light sleep, one stage of deep sleep and one stage of rapid eye movement (REM) sleep.

Light exposure of above 100 lux at night can lead to negative effects on sleep quality, and the transition between sleep cycles. In addition, light stimuli can lead to a shorter total period of time in the deeper, more restorative stages of sleep including REM sleep. 

Effects of artificial light on health

In modern society we often use electronic devices such as phones, laptops and tablets that emit artificial light. Using these technologies at nighttime can negatively affect our circadian rhythm. This is a relatively new problem in human history, following the advent of electricity in the 20th century. 

Excessive exposure to artificial light at night can interfere with circadian rhythm and have negative effects on human health, leading to complications such as:

In fact, one small study showed that just 100 lux of constant light at night resulted in higher heart rate and poorer blood glucose control. Therefore, it is important to limit light levels at night as much as possible.

Should I sleep in total darkness?

It is generally best to sleep in an environment that is as dark as possible, as pitch darkness helps to ensure your body is not signaled to wake up during the night. 

Even with your eyelids closed, light still can enter your eye and disrupt your sleep. Therefore, make sure your bedroom is as dark as possible before you go to sleep. Sleeping in a dark environment can have the following benefits: 

  • Reducing eye strain: a dark bedroom that minimizes ambient light can reduce eye strain.
  • Avoiding weight gain: light stimulus at night can interfere with your body’s metabolism, increasing the risk of weight gain. One study showed that women exposed to ambient light from a TV gained 10 pounds or more over five years. 
  • Reduced cancer risk: there may be a link between houses with high levels of artificial light and risk of cancer, according to some research. However, more research is needed to verify this effect. 

Due to the many benefits of minimizing light exposure whilst sleeping, it is important to keep your bedroom as dark as possible at night. 

How to keep your bedroom dark

Many people sleep in a bedroom that is penetrated by some form of artificial light, from electronic devices, intrusive streetlights, or the TV. However, it is important to reduce light as much as possible in your bedroom, to help you sleep. 

A few simple tips to make your bedroom more suitable for sleep include: 

  • Cover windows with thick curtains to block streetlight, e.g. using blackout curtains
  • Turn off electronic equipment that emits light
  • Wear an eye mask to further reduce light exposure 

Building a sleep routine around light

Because light is a powerful signal for sleep, it is important to build a sleep routine that involves a gradually darkening environment. This ensures ideal light conditions to align your circadian rhythm with the darkness at night.

This could involve the following steps: 

  • Install dimmer switches to reduce the amount of light you receive at night, and use the dim setting when the sun sets
  • Turn off electronic devices 2 hours before you plan to sleep 
  • If you must use an electronic device, turn down the brightness and turn on ‘night mode’ to reduce emitted blue light. 
  • Using a red light if you need to move around at night, as red wavelengths have been shown to be less disruptive to sleep

How to use light to optimize circadian rhythm

In addition to building a sleep routine around light, you can also ensure your circadian rhythm is healthy and aligned through monitoring light you receive during the day. The brightness and type of light, amount of timing exposed to light, and timing of exposure can all help to build a strong circadian rhythm. 

The following tips can help you to ensure you receive the optimal light exposure during the day:

  • Get 30 minutes of direct sunlight as soon as you can after waking up. Ideally, spend this time outside e.g. on the front porch. 
  • If you are unable to go outside in the morning, consider eating breakfast or drinking your morning coffee near a bright window.
  • Consider purchasing a light box to increase light exposure if you do not have access to natural light in your home. This is a form of light therapy using an electric light can help realign your circadian rhythm.
  • If you exercise during the day, try to do so outside if convenient to help receive more daytime light exposure.

Circadian rhythm sleep disorders

Circadian rhythm sleep disorders are a cluster of sleep disorders that are characterized by a misaligned circadian rhythm. These conditions may be caused by disrupted light signals during the day and at night, as well as shift work or flying across time zones.

Symptoms of circadian rhythm sleep disorders include difficulty falling asleep, fatigue during the day, and sleep deprivation. The most common circadian rhythm sleep disorders include: 

  • Delayed phase sleep disorder: most common in adolescents, this condition is when you go to sleep and wake up significantly later than what is typical. Generally, you have this condition if you regularly go to sleep at 2 a.m. or later. People with this condition may feel lazy and unproductive during the day, but energized and creative at nighttime. 
  • Advanced sleep phase disorder: this condition is when you fall asleep earlier than you would like (e.g. 6-8 p.m.) and wake up particularly early (e.g. 3-5 a.m.) on a regular basis. This condition is most common amongst the elderly. 
  • Jet lag: this involves your body’s circadian rhythm being synced with your departing destination’s time zone, rather than your current time zone. This can make it difficult to function in the new time zone, and you may experience gastrointestinal issues.


Light and dark has a powerful impact upon sleepiness, alertness, and a direct effect on the circadian clock. Your body responds to light signals to guide feelings of being awake or subjective sleepiness.

To ensure you have optimized your circadian rhythm, ensure to get light exposure during the day. At night, try to minimize the use of electronic screens 2 hours before sleep and ensure your bedroom is dark. These steps can help to ensure your circadian phase is aligned with the rising and setting of the sun, to promote optimal sleep. 

Improving your sleep quality and circadian system can prevent excessive body mass gain, and other negative health effects. 


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