The Sun Is Information: Why Light Governs Our Biology

Why Light Is Part of Our Biology, Not Just the Background of Life

Before nutritional supplements, laboratory tests, artificial lighting, and clocks, there existed a much older signal that every day told the human body when to wake up, when to be active, when to eat, and when to gradually begin preparation for sleep.

This signal is sunlight.

Today we are used to thinking about the sun mainly through two topics – vitamin D and the risk of sunburn. But light is much more than a source of UV radiation. It is information that reaches the brain, the nervous system, hormonal rhythms, sleep, and metabolism and helps cells understand what time of day it is.

Human biology did not develop under constant temperature, under LED lighting, and in front of a monitor. It was built in an environment with bright days, a changing solar spectrum, and dark nights.

And this difference matters.

The Body Has a Biological Clock

Almost every cell in the body possesses molecular mechanisms that follow an approximately 24-hour rhythm. These internal clocks, however, cannot simply operate independently – they must continuously receive information from the environment to stay synchronized.

The main circadian synchronizer is located in the suprachiasmatic nucleus of the hypothalamus, and one of the most powerful signals that reaches it is light.

In the retina there are specialized light-sensitive cells – intrinsically photosensitive retinal ganglion cells, or ipRGCs. Their main task is not to build the image we see. They transmit information to the brain about whether it is day or night and help the organism determine the phase of the day.

When morning light reaches the eyes, synchronization of an entire network of processes begins:

• the melatonin signal decreases;
• cortisol follows its normal morning rhythm;
• body temperature gradually rises;
• alertness increases;
• peripheral clocks in various tissues receive information that the active part of the day has begun.

That is why morning light matters far beyond the simple feeling that we have “woken up.” It participates in setting the entire biological day.

Good Sleep Starts in the Morning

At first glance it sounds paradoxical, but preparation for quality sleep in the evening begins right after waking up.

Bright daylight in the morning helps the brain set a clear boundary: now it is day. The stronger and clearer this daytime signal is, the better the circadian system can later organize the transition to the night mode.

That is why for a healthy circadian rhythm it is not enough for it simply to be dark in the evening.

What is needed is contrast between a bright day and a dark night.

And it is precisely this contrast that we are gradually losing.

During the day many people spend hours under relatively weak artificial lighting, and in the evening they continue with lamps, television, and phones a few dozen centimeters from their eyes.

This creates a strange biological environment:

The day becomes darker, and the night – brighter.

For the circadian system this is very different from the environment in which human physiology developed.

Outdoors and in the Office We Do Not Receive the Same Light Signal

Outdoors illuminance can reach tens of thousands of lux, while in a typical indoor environment we are often talking about only a few hundred.

For conscious vision this may be perfectly sufficient – we can read, work, and see without any problem.

But the biological clock does not ask the question:

“Can I see well enough?”

It receives information about the intensity, timing, and spectrum of the light.

That is why even an overcast sky outside can represent a much stronger circadian signal than typical office lighting.

This is one of the reasons why a short outing during the day matters, even when the weather does not look “sunny.”

Sunlight Is Not the Same Throughout the Day

The light environment changes continuously – morning, midday, afternoon, and at sunset. The human body developed precisely in this dynamic environment, not under the same spectrum from the moment of waking until bedtime.

Morning light is an important time signal. When the sun rises high enough, UVB radiation reaching the Earth’s surface allows the synthesis of vitamin D in the skin. The solar spectrum also contains red and near-infrared wavelengths that are actively studied today in the field of photobiomodulation.

In the evening the change is the opposite – the decreasing light gradually begins to inform the nervous system that the active part of the day is ending.

That is why light is not simply something that is on or off.

It is a language that the organism constantly reads.

Vitamin D Is Important, but It Is Only Part of the Story

This is the best-known connection between the sun and human physiology.

UVB radiation reaches the skin and participates in the conversion of 7-dehydrocholesterol to a precursor of vitamin D3. After further processing in the body, the forms of vitamin D that participate in numerous physiological processes are formed.

Vitamin D actually functions much more as part of a hormonal signaling system than as an ordinary vitamin. Its receptors are found in multiple tissues, and normal status is related to:

• bone mineralization;
• calcium balance;
• muscle function;
• immune regulation;
• a number of cell differentiation processes.

There is, however, one practical detail that is often overlooked: glass practically blocks UVB.

This means that you can sit by a very bright window and receive a light signal through the eyes, but that does not mean your skin synthesizes vitamin D in the same way as with appropriate exposure outdoors.

Being “in the light” and being outside are not the same.

The Sun Does Not Affect the Organism Only Through Vitamin D

If the only function of sunlight were to support vitamin D synthesis, we could simply take a capsule and close the topic.

But light interacts with the organism through other mechanisms as well.

One interesting example is nitric oxide. The skin contains compounds related to its metabolism and storage, and UVA exposure can help release some of these reserves.

Nitric oxide is an important signaling molecule for the vascular system and participates in:

• vasodilation;
• regulation of blood flow;
• endothelial function;
• control of vascular tone and blood pressure.

This is one of the reasons why modern photobiology views the effects of sunlight as a significantly more complex process than the simple formula:

sun → vitamin D.

Light Also Interacts with Mitochondrial Biology

Mitochondria are the structures in which a large part of the energy from nutrients is converted into usable cellular energy – ATP.

One of the key complexes in the electron transport chain is cytochrome c oxidase. It interacts with certain wavelengths in the red and near-infrared range, and it is precisely on such interactions that a large part of modern photobiomodulation is built.

In this area, potential influence on the following is studied:

• mitochondrial function;
• local blood circulation;
• oxidative balance;
• restorative processes;
• tissue adaptation.

It is important, however, not to turn this into an overly simplified idea that sunlight “charges mitochondria like a battery.”

Biology is much more complex.

It is more accurate to say that certain wavelengths of light can interact with cellular chromophores and thereby influence biological processes.

Light can be a biological stimulus.

Light Also Reaches the Brain, Mood, and Behavior

There is a reason why the dark winter months have such a strong influence on some people.

The light environment participates in the regulation of systems related to:

• serotonin signaling;
• dopamine signaling;
• alertness;
• mood;
• the circadian rhythm.

That is precisely why bright light therapy has an established application in seasonal affective disorder and is also being studied in other conditions.

But it is not necessary to wait for a mood problem to appear before we start thinking about light.

It is a daily part of the environment that regulates the brain – similar to nutrition, movement, and sleep.

Metabolism Also Has a Clock

Insulin sensitivity, hormonal secretion, liver metabolism, digestion, body temperature, and the use of nutrients show circadian variations.

This means that the same calorie does not enter an absolutely identical physiological context at 8 a.m. and at 1 a.m.

The calorie has not changed.

What has changed is the organism that must process it.

Light is one of the main signals that participate in organizing this context. That is why nutrition, sleep, and light should not be viewed as three completely independent topics.

They are parts of the same biological system.

The Problem Is Not the Sun. The Problem Is the Dose.

Here it is important not to go to the other extreme.

The fact that sunlight is an important biological signal does not mean that unlimited UV exposure is beneficial.

It is not.

Sunburn represents damage, and excessive UV exposure increases the risk of photoaging and various types of skin cancer. Repeated severe burns are of particular importance.

That is why the reasonable approach is not:

“The sun is dangerous – avoid it.”

But it is also not:

“The sun is beneficial – the more the better.”

The more accurate principle is:

Regular and individually adjusted exposure without burning.

How Much Sun Do We Need?

There is no universal number of minutes that is correct for everyone.

Appropriate exposure depends on multiple factors:

• geographic latitude;
• season;
• time of day;
• cloud cover;
• UV index;
• skin color;
• age;
• area of exposed skin;
• certain medications;
• individual photosensitivity.

That is why the rule “20 minutes of sun is enough for everyone” is overly simplified.

It is much more practical to think of light as part of the structure of the entire day.

Morning: Give the Brain a Clear Signal That the Day Has Begun

One of the simplest strategies is to go outside relatively soon after waking up.

It is not necessary to look directly at the sun.

Simply be outdoors.

This can be:

• a short walk;
• coffee on the terrace;
• a few minutes of movement;
• part of the way to work on foot.