Why is light important to get a good night sleep?

Human life started near the Equator. The length of day and night did not change much with the seasons So we used this light and dark periods to help our sleep wake up cycle - the circadian rhythm. Still humans are the only animal species that can manipulate circadian rhythm by staying up all night for safety or hunting.

Then fire was discovered. This increased social interaction at night time. Improved nutrition through the cooking of food. Then Humans could start to travel into colder climates with shorted seasonal days as they had heat to warm them. With this we had to start to get used to shorter days and nights which disrupt our circadian rhythm. In fact this is one of the reasons we get SAD (seasonal affective disorder) today. We’re just not built to Handel these changes very well.

So why is light so important for these rhythms and how can it help with a good night sleep?

Human eyes are like a camera. They contain millions of rod and cone cells that capture the details of an image in fine resolution and send this info through long wire like nerve cells to the brain. The retina the light sensitive tissue at the back of the eye contain several million rod and cone light sensors. Light rays are focused onto the retina through our cornea pupil and lens. The retina converts the light rays into impulses that travel through the optic nerve to our brain where they are interpreted as the images we see. The idea of this blows my mind!

In 2002 three independent research groups discovered a light sensing protein present out side the rod and cone cells that is in fact the light sensor that entrains the daily sleep wake cycle to light. The light sensing protein is called melanopsin.

In mice studies those without melanopsin cells in the dark maintained normal circadian rhythm however they found it harder to adjust to their sleep wake up time to the light-dark cycle. Normal mice adjusted within a week. Non melanopsin mice took 1 month. These mice also lost their ability to freeze to bright light.

Light colours

Visible light includes all the colours of the rainbow each colour has a different wavelength. Red has the longest wavelength and violet has the shortest. When all the waves are seen together they make white light or sunlight. Melanopsin protein is most sensitive to blue light waves and less sensitive to red light. When melanopsin is activated by registering blue light it sends a signal to the brain that light is present and the brain responds by thinking it is daytime regardless of what time it really is. If you walk into a supermarket at night your melanopsin is registering the overhead light and your brain thinks it is daytime and that you should be awake.

These discoveries help us begin to understand how light affects health. Our modern lifestyle in which we spend most of our time in doors looking at bright screens and turn on bright lights at night activates Melanopsin at the wrong times of day and night which then disrupts our circadian rhythm and reduces the production of the sleep hormone melatonin as a result we cannot get restorative sleep. When we wake up the next day and spend most of our day indoors the dim indoor light cannot fully activate Melanopsin which means that we cannot align our circadian clock to the day night cycle making us feel sleepy and less alert after a few days or weeks we get into depression and anxiety.

Now that we have a better understanding of the quality quantity and duration of light that can shape or break our health we can begin to imagine how simple changes to our lightbulbs computer screens or eyeglasses can go along way to restoring or improving our health.

So the simple rules to follow to help you circadian rhythm are:

no screens past 10pm and definitely not if you wake up in the night.
in the morning get outside for 10 mins. If the melanopsin proteins get morning light at the right time this sets the circadian clock and will help you produce melatonin (which makes us sleep) at the right time before we go to bed.