The circadian rhythm controls most body functions

Light therapy

Every living thing has a number of internal rhythms that control many functions. Some rhythms are short, such as a cycle of sleep that lasts 60-120 minutes. Other cycles are longer such as the menstrual cycle. The circadian rhythm has a cycle length of around 24 hours, and is the rhythm that controls most body functions, ensuring that things work in the same way at the same time each day. The most obvious things the circadian rhythm controls are wake, sleep, activity and appetite. However, recent research has shown that almost all body functions are regulated by the circadian system. Light can be used in treating circadian rhythm disorders as part of circadian rhythm management.

What role does light play in regulating the circadian system?

Although our internal body clock has a cycle length of approximately 24 hours, most people have a cycle length of a little longer than 24 hours. The average for humans is around 24hrs 15mins. In reality, that means when the alarm goes off each morning, exactly 24 hours after it went off the day before, our internal clock thinks there are still 15 minutes left before the start of the next ‘day’. That is why, for most of us, the alarm always feels that it goes just that bit too early. The exception is the around 3% of people who have an internal cycle length of less than 24 hours. These are the people who wake early, often before the alarm and function best early in the morning. Most of us, the other 97%, can take a while to get going in the morning.

Light helps to synchronise, or reset the circadian system. All humans have receptors in the eyes that are involved in detecting light as a signal to reset or synchronise our internal clocks to match with day and night in the world we live in. These cells are most sensitive to blue-green light wavelength 459–485 nm. Exposure to this sort of light of sufficient intensity when our alarm goes in the morning can ‘reset’ our internal clock so it knows it is time to wake up and start the day, rather than expecting that to be 15 minutes later each day if our internal clock has a cycle length of 24hr 15min.

In experiments done in the late 1930s and 1940s, early sleep researchers spent months in caves, completely removed from any sunlight or other light signals to indicate day or night. They found that they slept and woke up about 15 minutes later each day, showing that their internal clocks had a cycle length of longer than 24 hours, and in the absence of light, this rhythm wasn’t reset to Earth’s day, night cycle.

How can light be used to manipulate the body clock?

Light of a wavelength 459-485nm, first thing in the morning, shifts the body clock to an earlier time and prevents the clock drifting gradually later each day. So, if people have trouble getting up in the morning, having light exposure can help with getting up earlier.

Light in the evening, gives the circadian rhythm a signal that the sun hasn’t gone down yet, and delays the onset of sleep, pushing the internal clock to a later time. Devices such as tablets and smartphones, can have emit enough light to trick the internal rhythm in to thinking it is still daytime and make it harder to get to sleep. For more on the impact of screens on sleep see this post.

Light therapy has been used in three common disorders:

  • Jet-lag: When travelling, making sure light exposure matches that of your destination can help to adjust to your new time zone (Post on managing jet lag)
  • Insomnia: For around 40% of people with insomnia, and maybe more, there is a component due to the circadian rhythm not being synchronised with the day night cycle
  • Depression: Seasonal affective disorder (SADs) or winter blues have been managed with light for many years, but in the last decade the role of light in non-seasonal depression has been increasingly recognised (Post on seasonal affective disorder)

Using light therapy does require some guidance from your health professional, so if you think light therapy may help you or are interested in knowing whether it may have a role, talk with you health professional.

What sort of light can be used?

The wavelength of light that works best is in the blue/green spectrum at a wavelength of 459–485 nm. Depending on the mix of wavelengths this can look blue (eg GoLiteBlu) or green as in Re-Timer or Feel Bright Light. Full spectrum white light, such as outdoor light, contains the right blue/green wavelength for shifting the body clock, but many indoor lights create a light that looks white but contain very little blue/green spectrum light.

There are a number of light sources on the market. Here are 3 that are available in Australia:


Re-Timer glasses (AUD$299): These have a nice looking design and are comfortable to wear, but are somewhat flimsy, so you need to be careful with them. To protect them, they come in a hard case, which is pretty big. That’s an issue when using these for jet-lag as it they can take a lot of room in your carry-on. Despite this, these are my current favourite because of their ease of use, and they are comfortable to wear. They are available only from Re-Timer directly.

feel bright lightFeel Bright Light (AUD$250): These aren’t as elegantly designed as the Re-Timers, and the light source (small black box attached to visor) needs to be attached either to the supplied visor or a baseball cap. I find these easier to read or work on a laptop whilst using as the light was comes just above the eyes whilst I was looking slightly down. The light unit is also more robust so I would be more comfortable putting it in my carry-on without being worried it would get broken. You do have to wear a visor/hat with these. They are available online.

Philips-goLITE-BLUGoLiteBlu (Around US$200): The GoLiteBlu is designed to sit on a desk, around 60cm from the eyes, rather than be worn like the above 2 devices. This means that you do need to be sitting at a table or desk to use the GoLiteBlu. People often find this a little less practical, particularly in the morning when they may not have the time to sit for 20-30 minutes. Currently they are not readily available in Australia, so can be ordered from Amazon or Ebay and shipped to Australia.

What about blocking light at night?

As well as using light in the morning, it is important to reduce exposure to blue wavelength light at night as it can make it harder to get to sleep. This is discussed in more detail in this post, and some of the devices and tools used are listed below.

Blueblock glasses

Uvex blue light blocking glasses (AUD$25): As well as using light in the morning, blocking light in the evening is an important. Too much light after the sun has gone down can delay the onset of sleep. Computers and tablets produce blue light and are often close to the eyes, so can make it harder to switch off at night. Using blue light blocking glasses, such as the Uvex glasses  can help.

There are also apps available, such as f.lux that reduce blue light from computer screens and tablets or phones, and current versions of iOS (9.3 or later) have a function Night Shift that reduces blue wavelength light emitted by devices after dark.

Smart lighting systems of the future

With increasing recognition of the impact on light on sleep and health, companies are beginning to produce lights for houses and workplaces that are capable of changing wavelength at different times through the day, and can be adjusted from a smartphone app. The globes can be set to produce colder, blue colours in the morning and warmer, yellow-orange colours at night. One example of this technology are the C-Sleep globes from GE.

Research is currently being undertaken looking at the effects of smart lighting systems on work productivity, particularly in shift workers, as well as the effects on sleep and health. It’s likely that in the future lighting in our homes and workplaces will change wavelength across the day to optimise sleep, health and work performance.

Related posts & links:

Need more information about how you can sleep better?

At Sleephub we understand the struggle people endure with sleeping problems which is why we have created a comprehensive FAQs page with information for those seeking information about sleep disorders and potential solutions.

Check our resources or take our Sleep Wellness Quiz for a free assessment of elements that may be keeping you from a good night’s sleep.

Recommended Posts
pingbacks / trackbacks
  • […] Cunnington, D 2015, ‘What is Light Therapy?’, SleepHub, 21 May, viewed 20 June 2018, […]

Tell us what you think