How light impacts your mood, learning, eating, and sleep (Part 1)

How light impacts your mood, learning, eating, and sleep (Part 1)

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How light impacts your mood, learning, eating, and sleep (Part 1)
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Plus 4 fun and informative experiments you can do today

You’ve heard about how using your phone an hour before bedtime can affect your sleep. But did you know that light impacts so much more than just your sleep?

In this 2 part series, we’ll be exploring how light affects your mood, learning, eating, and sleep by showing you how to determine your light sensitivity. When you know this you’ll be able to create your optimal sleep-wake cycle in line with your circadian rhythm.

In this article (Part 1), we’ll look at:

  • What is a circadian rhythm
  • How your circadian rhythm affects your mood and energy
  • How light affects your sleep-wake rhythms
  • How to interact with light at different stages of the day
  • 2 Experiments to test your light sensitivity and reset your circadian clock

In Part 2, we’ll show you how to regulate your mood and appetite by timing your food intake and exercise perfectly for your body and mind and give you tools to deal with:

  • Jetlag
  • Shift work
  • Resetting your clock after a late night out

So let’s start with understanding what circadian rhythm is…

What is a circadian rhythm and how does it affect your mood and energy?

Circadian Rhythm is your body’s clock for daytime and nighttime. When you open your eyes in the morning you see light. This conscious vision tells your body and mind that it’s daytime.

But circadian clocks work independently of your conscious vision. It happens in your subconscious vision. This means that even blind people with no conscious vision are affected by light. Removing a blind person’s eyes would cause sleep problems as they would have no circadian rhythm.

Put simply, your circadian clock tells you what time it is for waking up, eating, and sleeping.

The word circadian comes from two words; circa (approximate) and Diem (day). When put together, circadian means approximate day. You should know that your natural circadian clock does not work for 24 hours as a solar day does.

Dr. Samer Hattar, an expert on light and circadian rhythms at the National Institute of Mental Health, explains that a human’s typical Circadian Rhythm is set to around 24.2 hours. This means your internal clock shifts 0.2 hours every day. He goes on to say that by getting the right amount of sunlight every day you can shift your circadian rhythm back in line with the 24-hour solar day.

Another way of looking at this is to imagine you’re in a dark cave for a very long time with no access to light. You would still sleep and wake at regular times (set by your circadian clock). But your mind and body would be experiencing slightly longer days (24.2 hours) than the actual solar 24-hour day.

And why is aligning your circadian clock with the solar clock important?

Imagine you’re off for 0.2 hours a day. Within 5 days you will be out by an hour. In two weeks you’ll be out by nearly 3 hours. You’re 3 hours out of phase with the rest of your society. As you fall further and further from the solar clock, you’ll feel more and more “jetlagged” despite never having traveled.

Make sense? Great. Now that you understand how your circadian clock works, we’ll move on to how light impacts your mood and energy levels…

How light affects your sleep-wake rhythms…

Light intensity is important here. Your conscious vision is not very good at measuring light intensity. But your subconscious vision (your circadian rhythm) is.

I want you to imagine for a moment that your eyes are a camera. You can set a camera to take photos in dark conditions. A camera can also be set to take photos in very bright conditions. If you use the dark setting in bright conditions you will end up with a completely white photo.

But because your conscious vision doesn’t measure light intensity well, it can be difficult for you to understand if you are getting enough natural light to align your circadian rhythm.

How to measure your individual light sensitivity

We’ve already discussed how your conscious vision is not good at measuring light intensity. So it can’t be trusted to tell you if you are getting enough light. Here’s a fun experiment you can do to measure your light sensitivity…


At night time, when there is no natural light, be in a room with multiple lights turned on. Try to make this the normal amount of lights you use in the evenings and at night.

Let’s say you use 5 lights – 3 ceiling lights and two lamps. Try turning 2 of these lights off. You will immediately sense a difference in the light. But after 15 minutes check to see if you can still sense the difference. Are your eyes straining to see? If they aren’t straining with less light, you are exposing yourself to too much light (you are more sensitive to light).

Keep trying this experiment until you’re using the lowest number of lights without having to strain your eyes. That’s the comfort level of your light sensitivity. Most people need probably 10x less light in the nighttime than they need later in the day.

To better understand how light affects your sleep-wake rhythm, let’s break a typical day down into 4 parts and see how you should be interacting with light at each part.

For this example, we’ll assume you wake up in the morning with the sunrise…

How to interact with light in the morning after you wake up

Get as much light as you can first thing in the morning to set your clock to solar time. Get outside, and see the natural light daily for around 15 minutes or more – you don’t have to see the sun, just the natural light. If it isn’t sunny, you should stay for 30-45 minutes to get the light you need.

Should you use sunlight simulators?

If you live in the north and don’t get a lot of natural sunlight when you wake up, you can also get the right amount of light from artificial light. Getting plenty of light in the morning will help you adjust your cycle to the day-night cycle. Your circadian clock in your brain is always trying to work out when you are on time.

Are you an early bird or a night owl?

Do you like to wake up early in the morning or do you prefer to stay awake later at night? You may think you know who you are. But there is only one way to find out.


You have to wake up early, expose yourself to the light as described above, and see how this makes you feel. From this, you’ll understand if you are better off waking up early or going to sleep late. FYI – most people who think they are ‘night owls’ actually end up being ‘early birds’. Try this experiment yourself and see what results you get.

How to interact with light in the middle of the day

If you get enough light in the morning, you don’t necessarily need a lot of light during the day. But it is still good to get light in the middle of the day. Maybe take a walk outside during your lunch break.

How to interact with light in the evening

The best thing to do here is to let the natural light creep into darkness. Watch the sunset. Allow your circadian clock to set itself to the natural solar day-night cycle. If you watch the sunset until it gets dark, your body and mind know it is nighttime.

But if you stay indoors and turn on tons of artificial lights blocking out the dimming of the natural light, your circadian clock gets confused and thinks it is still light hours.

Should you use Blue blockers in the evening?

Blue blockers can be dangerous. Blue blockers are effective at blocking out the blue light from artificial light sources in the evening and this helps your circadian clock. However, according to Dr. Samer Hattar, they are not good to use in general.

He states that they are distorting your vision. You will experience a heightened sensitivity to yellow lights. This affects the spectrum of light that you can see even when not using Blue blockers.


Instead, he suggests you dim the blue light and increase the yellow light. You can do this on most modern mobile phones in the brightness and display settings. Some phones offer a “Nightshift” option which will warm the colors on your screen automatically (reducing but not eliminating the blue light)  between a set time. This should be set between sunset and sunrise.

How to interact with light at night

There are dangers in exposing yourself to light between 10 pm and 4 am. Using the lowered blue light settings on screens can improve your exposure to blue light which affects your circadian rhythm in the evening and nighttime.

But the best option is to keep it as low light as possible without using screens. If you do use screens, use them with the lowest brightness setting.

How to reset your circadian clock if it’s out of phase


Completely removing artificial light for 2 days and exposing yourself to natural light and natural dark will reset your circadian clock. If you are out by many hours, you may need more days. If you continue to use but limit as much as possible your exposure to artificial light (computers, phones, industrial lighting) it will take closer to 7 days to reset your circadian clock.


So we’ve reached the end of Part 1 in our ‘How light impacts your mood, learning, eating, and sleep’ 2 part series. You’ve learned how to measure your light sensitivity, how to reset your circadian clock, and how you should be interacting with light throughout the day.

Start using these suggestions over the next few weeks and read part 2 (coming soon) to discover how to regulate your mood and appetite and learn 3 tools you can use to deal with jetlag, working shifts, and resetting after a late night out.

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