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On a usual Saturday,
you wouldn't find me here.

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And I mean, of course, I don’t give
TEDx Talks every Saturday.

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But what I mean is, you probably
wouldn't find me in a museum.

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Rather, if you were looking for me,

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you'd probably have to go
to the mountains,

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and that is because
I absolutely love hiking.

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And in addition to just being outside
and being in nature,

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what I really like is this feeling
of physical exhaustion,

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sort of this satisfaction
you can feel in every single muscle.

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And in the evening,
when it's time to go to bed,

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I absolutely cherish
this amazing sleepiness

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that fills you from head to toe.

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Now you might say, "Well, of course
she's tired. She's been hiking all day.

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Plus, not to forget about the effects
of supposedly fresh air."

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But there is one factor
that most likely outweighs

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the effects of fresh air
and perhaps even physical activity,

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and that is exposure to natural daylight.

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And today, I hope to convince you
that we should all appreciate daylight

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as sort of a natural soporific.

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And hopefully, by the end of the talk,

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you will also recognize
that we should all pay more attention

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to our daily light diet.

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But let's start with the question
why daylight is so important

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for our organism, for health
and, eventually, for sleep.

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Now throughout your body,
in each and every single cell,

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there is a tiny molecular clock
ticking inside it.

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And to keep these millions
of clocks in sync,

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we have one central body clock
that is located in the brain,

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in an area which we call the hypothalamus.

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And like the conductor of an orchestra,

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it communicates the time of day
to each of these molecular clocks.

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And this way, it is able
to regulate bodily processes

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in sync with the time of day.

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For example,

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it causes your body temperature
to increase in the morning

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and to decrease in the evening.

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It choreographs the release
of certain hormones at appropriate times.

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And in the evening,
when it’s time to go to bed,

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it will also make you tired and sleepy.

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But given this precise synchronization
between internal or biological time

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and external time or environmental time,

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it seems clear that the body clock

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cannot be blind or isolated
or shut off from the environment,

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but rather, it needs to receive
information about the time of day

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from the environment

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for it to synchronize with sun time.

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And this is achieved by close connections

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between the internal biological clock
in the brain and our eyes.

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And now you may know
that in the human retina,

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there are different types of receptors,

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so photoreceptors,
so receptors that sense light.

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

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Now the rods only contribute
to a visual impression

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under very dim lighting conditions.

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And here, in the background,
you now see a picture

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that might, more or less, be brought about
by only the involvement of the rods.

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And what you can see
is that it's only shades of gray,

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it's slightly blurred,

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and around the point of fixation,
which is indicated by the gray dot here,

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you have a little scotoma,

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so an area where you can't
really see anything.

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Now who recognizes what that is?

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Yeah, excellent.

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But it's going to be way easier,

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and the majority of you
will recognize what it is,

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when I now switch to the next slide,

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which is brought about
by the involvement of the cones.

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Of course, it's the town hall of Basel,
but now you can appreciate the colors,

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fine details.

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And if this was animated,
you'd even see fine details in motion.

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But this is not the whole story,
because only fairly recently,

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only in the early 2000s,
another type of cell has been discovered,

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and we call them retinal ganglion cells.

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Now, you might ask yourselves,
"What picture is she going to show next?"

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But I'll have to disappoint you ...

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I’m not, and I actually can’t,
show another picture.

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Because from all we know,

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these cells do not contribute
to a visual impression.

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But they are exclusively designed

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to sense short wavelength
proportions in daylight.

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Sometimes, we also call this blue light.

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So they're designed to extract important
information about the time of day

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from the environment

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and pass this on
to the internal biological clock

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in the brain.

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And I guess you've all experienced
how well this biological timing system,

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this connection between our biological
clock and the external world, or our eyes,

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works when we, for example,
travel across time zones.

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Now how much light do we actually need?

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How much light is enough
for the positive effects on, for instance,

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sleep to occur?

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

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But I think what we have
to keep in keep in mind

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is that the biological timing system
has evolved under the open sky

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and not in offices or museums.

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

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Office light or light in rooms

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is in no way comparable
to what we find outside.

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And therefore,
from a scientific perspective,

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I can only recommend you to spend
as much time as you can

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under the open sky,

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but try to make it at least
30 minutes per day.

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Now let's finally talk about sleep.

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And I've repeatedly alluded
to the fact that daylight is beneficial

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and important for sleep.

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And in this context,
I'd like to share a little story.

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So last year, I had to go
to hospital for surgery.

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And generally, the hospital environment
is a very challenging one for sleep.

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Because you spend way too much time
in bed, not moving much.

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You might be in pain.

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Now and then, someone comes in
to check on you, even during the night.

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And if you're as lucky as I was,
you have a snoring roommate.

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(Laughter)

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And because you, of course,
don't get a lot of daylight.

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And that's why many people,
while they're in hospital,

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ask for sleep medication.

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But as I consider myself
to be a generally good sleeper,

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and also because I know
how these drugs work,

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I didn’t want this.

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But I knew there was something I could do
to help my body sleep as well as possible,

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even in this situation.

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And that was to maximize
exposure to daylight.

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Because as a sleep scientist,

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of course, I know
about the research findings.

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I know that the more
daylight that you get,

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the more tired you will be in the evening,

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the easier it will be
for you to fall asleep

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and the better the experienced
sleep quality.

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And in addition,

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daylight exposure has also been shown
to increase proportions of deep sleep,

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which again has been linked
to processes of tissue repair,

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something not to be
underestimated after surgery.

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And beyond the effects on sleep,

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we know that patients in brighter rooms
experience less pain and less stress.

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And also the effects of daylight
on mood are well established.

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So I thought it might be time for me
to put into practice what I often preach.

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And I have to confess that usually

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I'm much better at giving advice
than following it myself.

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But here's what I did.

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So first, when it became free,

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I asked the nurses to move
my bed to the window place,

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because that does not only give you
power of the fresh air supply,

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which is highly recommended
in the hospital,

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but it also maximizes daylight exposure
even though you're inside.

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Second, as soon as I could
leave my bed again,

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I went to the park for short walks.

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And last, every meal I had,
I tried to take on the little balcony.

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Did it work?

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Well, it's a bit difficult to say,

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because it of course
was a single-case study.

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But from patients suffering from insomnia,

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so one of the most common sleep disorders,

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we know that light therapy is beneficial.

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And therefore, I would like
to encourage all of you,

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and perhaps especially those
who might belong

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to the 25 percent
suffering from sleep problems,

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to start your own single-case study.

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Thank you.

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(Applause)