Christine Blume

And this way, it is able to regulate bodily processes in sync with the time of day.

For example, it causes your body temperature to increase in the morning and to decrease in the evening.

It choreographs the release of certain hormones at appropriate times.

And in the evening, when it's time to go to bed, it will also make you tired and sleepy.

But given this precise synchronization between internal or biological time and external time or even environmental time, it seems clear that the body clock cannot be blind or isolated or shut off from the environment, but rather it needs to receive information about the time of day from the environment for it to synchronize with sun time.

And this is achieved by close connections between the internal biological clock in the brain and our eyes.

And now you may know that in the human retina, there are different types of receptors, photoreceptors, so receptors that sense light.

And classically, we distinguish two types, the rods and the cones.

But this is not the whole story.

because only fairly recently, only in the early 2000s, another type of cell has been discovered, and we call them retinal ganglion cells.

These cells do not contribute to a visual impression, but they're exclusively designed to sense short wavelength proportions in daylight.

Sometimes we also call this blue light.

So they're designed to extract important information about the time of day from the environment and pass this on to the internal biological clock in the brain.

And I guess you've all experienced how well this biological timing system, this connection between our biological clock and the external world or our eyes, works.

When we, for example, travel across time zones,

Now, how much light do we actually need?

How much light is enough for the positive effects on, for instance, sleep to occur?

And I have to admit, this is not so easy to answer.

But I think what we have to keep in mind is that the biological timing system has evolved under the open sky and not in offices or museums.

So it is also optimally tuned to the conditions we find outside.