There is more to light than meets the eye

Summer sun

Light is essential for our circadian clock!

All living organisms, you and I included, have biological rhythms in physiology and behaviour. Our most obvious behavioural rhythm is our daily pattern of sleeping and waking. Other biological rhythms include daily changes in body temperature, hormone secretion, appetite and alertness.

Scientists have shown that these daily rhythms are more than passive reactions to the environment; they are generated by an internal body clock – the circadian clock. Each person’s circadian clock has an innate rhythm that is close to, but not exactly, 24 hours (if you are a late type like me, chances are your clock is a little longer than 24 hours). If you were to stay in a cave for a number of weeks without access to the outside world and knowledge of local time (yes, scientists have tested this!), you would continue your daily rhythms, but these rhythms would slowly drift out of sync with the outside world.

The clock synchronizes to the outside world by means of time cues in the environment, the most important being the light/dark cycle of the sun. A study by the Ludwig-Maximilian University of Munich shows just how important daylight is in the timing of our sleep. While Germany has the same time zone across the country, the timing of its residents’ sleep on free days varies systematically from east to west by about 30 minutes, matching the difference in the timing of sunrise.

When we don’t get enough daylight, our circadian clock fails to properly synchronise, making it vulnerable to drift, just like in a cave.

Light impacts the circadian clock via direct nerve connections between our eyes and the brain – not, as many assume, the skin. So for light to yield its positive effects, you must allow your eyes to absorb the light. If you are tanning at the beach with a cap over your head, you are in the dark, circadianly speaking! So is modern day indoor life. Indoor lighting levels (generally below 400 lux) are extremely low in intensity compared to outdoor light (about 20,000 lux in the shade on a clear blue sky day or about 100,000 in bright sunlight). When we don’t get enough daylight, our circadian clock fails to properly synchronise, making it vulnerable to drift, just like in a cave. This is of particular concern to city dwellers. Research shows that people’s sleep is much more uncoupled from the light/dark cycle in urban areas (population size > 500.000) than in rural areas (population size < 300.000), where people generally spend more time outdoors.

Moreover, receiving light at the wrong time of day (light pollution) can disturb the circadian clock. Artificial light at night, from indoor lighting to electronic devices (e.g. computer or phone screens) suppresses the release of melatonin, essential for sleep. It can also lead the clock to synchronise to the artificial night light instead of daylight (in particular when the latter is in short supply), potentially delaying our sleep into the early morning hours.

Circadian clock disturbances are more than an annoyance! They are closely tied to sleep and mood disorders, unhealthy lifestyle choices (cigarette and alcohol consumption) and in the long run, can have serious consequences to our health, significantly increasing our risk for heart disease, digestive disorders, obesity, diabetes, reproductive problems, and even cancer.

The good news is that you can do something about it. Leave your cave. Or check out

Author: Dr. Myriam Juda

Dr. Myriam Juda is a research psychologist specialized in the sleep-wake cycle and work imposed circadian disruptions such as work schedules and lighting. She is the founder of Circadian Light Therapy (

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