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Good morning, Vienna.

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Okay, many cultures use
the expression 'night and day'

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to describe opposites
that are somehow surprising:

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Twins that behave completely differently
despite that they look the same;

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someone who's drastically
changed their behaviour,

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perhaps going from drinking
way too much to nothing at all.

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'It's like night and day'.

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Of course when we say this,
it's actually a metaphor

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that refers to the stunning transformation

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that the Earth goes through
every day as it rotates on its axis,

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alternately sampling
warmer light environments

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and darker cold ones.

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It's like night and day, there's nothing
as different from night than day.

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And we pass between these two states
every single day without exception.

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Evolution is all about developing
adaptations to special environments

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through genetic change.

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And so it stands to reason evolution
has given us a mechanism

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to deal with this special environment,
a mechanism embedded in our genes,

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and a mechanism that actually samples
and tastes the environment

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that's alternating
with high predictability.

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And that mechanism
is the biological clock,

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or the circadian clock.

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So what is the circadian clock
and what does it do for us?

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One of the most impressive
examples that I know about

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are experiments
that take people like you or me

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and put them into a time-free environment.

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A subject goes into
a specially designed apartment,

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usually just a single room,

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that has no access to information
from the outside world.

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So there's no clock on the wall,
there's no window

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and there's no noise
from outside activities.

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In this situation, a subject lives
not with a precise 24-hour rhythm,

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they don't maintain that,

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but they go to a circa-24-hour rhythm
in sleep-wake behaviour, for instance.

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They basically sleep and wake
with the same interval every day,

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but it's a little bit less than 24 hours
or a little bit more than 24 hours,

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so it's a persistent oscillation
in our behaviour

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in the absense of daily time cues.

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And this is called the circadian rhythm.

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With the word 'circadian'
coming from the Latin

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for 'about a day'.

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So -

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so I think about how this works,

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that you have no information from outside
and you still can do this quite reliably

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in constant conditions.

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This is what we mean by an internal clock,
a biological clock or a circadian clock.

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So it's an alarm clock
that's built into our heads

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that's telling us when to wake up.

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And then I have to say that
I've been studying the circadian clock

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for over 20 years now,

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and I still am incredibly
impressed and amazed,

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and it's still almost unbelievable to me

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that we, who are so complex
and strong-willed in our behaviours,

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show this persistent,
self-sustained circadian rhythm

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in sleeping and waking, for instance,
and in many other things also.

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But I've seen so many examples
of this 24-hour timing mechanism

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in bacteria and fungi and plants
and humans and other animals

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that there's no question in my mind
that it's a fundamental part of biology.

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So as you probably know,

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experiments that put humans
or any other living organism

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into a time-free environment
like I just described

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are actually highly artificial,

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and they actually never
really happen in real life.

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And you also probably know
that your sleep-wake cycle

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is actually, on average, exactly 24 hours.

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This process of adjustment
of this circa-24-hour rhythm

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to exactly 24 hours is called
circadian entrainment,

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and for our sleep-wake behaviour

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it's mediated by specialised cells
in our retina that sense light.

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The biological clock, however,

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is built of many more cells than just
a few cells in the eye or the brain.

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Basically, all of our cells
are oscillating.

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What do these cellular
oscillations look like?

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Well, we can measure oscillations
in RNA levels, for instance.

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The genes express your RNA levels
in individual cells,

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and you see here a tracing of cells

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that are carrying on
for five or six or seven days

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in constant conditions.

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So about ten percent
of the genes in any given cell

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are expressed with a circadian rhythm.

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Of these, some of the proteins
will also be rhythmic,

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and metabolism will become rhythmic also.

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And so you get a network
of oscillations in each cell

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that eventually come together

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so that you get higher functions
that are also rhythmic.

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Now, these cellular oscillations
are generally not entrained by light;

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light entrainness is unique to cells
emanating from the eye, in the eye,

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or connected to the eye, those cells.

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These peripheral cells, in general,

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will entrain to cycles
in our body temperature,

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so our temperature is one to two degrees
higher at the end of the afternoon

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than it is at the end of the night.

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So now you should start
to get a picture of the circadian clock

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in a human, for instance.

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It's all kinds of oscillating cells -

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really, literally, all kinds
of cells that are oscillating.

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They're responding to various time cues:

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light, temperature,

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also nutritional status
and hormone levels, for instance,

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and then they come together
to regulate many, many processes.

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So cognitive performance changes
over the course of the day.

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The perception of pain
peaks in the evening.

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Physical performance like grip strength
peaks mid-afternoon,

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and components of our blood

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like the metabolic state of cells
or hormone levels

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is also circulating with predictable
rhythms over the course of the day.

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Andre Gide wrote
a wonderfully perceptive sentence:

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'... if I were not there
to make them acquainted,

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my morning's self
would not know my evening's.'

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This is really perfect -
we are different people, night and day.

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So, what does this mean
for us in everyday life?

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The circadian clock regulates
the timing of our behaviour.

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It determines our chronotype.

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Chronotype is measured
by asking people when they sleep

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on a free day with no obligations.

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And like many behaviours,
it shows a distribution.

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You get a few people who are sleeping
very early in the day;

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you get most people who are sleeping
at similar times to each other;

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and then you get some people
who are sleeping quite late.

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(Laughter)

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And yes, you can see
that teenagers and young adults

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are disproportionately represented
in this late population.

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They sleep much later
than young children and older adults.

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That makes me think back
to actually what Gide wrote

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about being different people
morning and evening

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and I think that this explains
a lot of the conflict

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between parents and teenagers -

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that basically, the timing
of their behaviours is totally misaligned,

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that's all it is.

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(Laughter)

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Okay, so ...

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So I told you about age
and how that regulates chronotype,

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and so think about this also,
that as you go through life, then,

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you naturally pass through
a series of chronotypes

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going from early to late
and then early again, okay?

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So what else regulates chronotype,
or the timing of our behaviour?

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One of the answers is genes.

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And the most compelling -
not the most compelling, excuse me -

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the first data that informed us
about the genes

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that are involved
in regulating the circadian clock

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came from fruit flies,

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and we now have a lot of genes
that regulate the circadian clock

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in isolated human cells.

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Interestingly, when we look
at humans themselves,

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it's much more difficult to figure out

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which genes are involved
in regulating chronotype.

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The most compelling examples
come from large family pedigrees

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where an extreme chronotype
is inherited as a dominant trait.

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However, the DNA sequences
responsible for those traits

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are actually not found
in the general population,

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so my conclusion from these observations

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is that the genes that are involved
in regulating chronotype

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are actually many,

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with each having individual small effects
in the general population at least,

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and therefore very hard to track
by standard genetics methods.

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Another way that the chronotype
is regulated is with light.

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So I mentioned that the clock is designed
to actually sample the environment,

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so perhaps it's not surprising
that it actually checks in

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and uses some of the information
from the environment.

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So, how do we know that this is so?

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If you live in a higher light environment
compared to a lower light environment -

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so someone who gets outside compared
to someone who's inside all the time -

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you'll generally have an earlier
chronotype vs. a later one, respectively.

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If you live in the eastern part
of a time zone like in Vienna

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compared to the western part
of the time zone,

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you'll have an earlier
vs. a later chronotype,

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and of course this is because
we express chronotype as local time,

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whereas your clock is reading sun time.

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There's some evidence
that we entrain differently

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in the summer -
that's earlier in the summer -

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than in the winter,
where we entrain later.

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It might also help you
to think about jet lag

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to understand intuitively
how light feeds into the clock.

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So with jet lag
you've flown across time zones,

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and it takes a really
surprisingly long time

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to readjust to the new time,
and you feel really bad in the process

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as your clock is running ahead
or lagging behind

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of the new sun time.

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Eventually you find a relatively stable
phase of circadian entrainment

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that's consistent for your chronotype
in your new environment.

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You feel better; you can sleep again;
and you get your energy back.

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So, what are the implications
about all of this for you

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in your everyday life?

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First of all, I think it's pretty clear

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that you will have a fundamentally
different circadian clock,

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and therefore chronotype,
from your neighbours.

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So you have different genes,
you have a different age,

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and you have a different life history.

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I want to do a little experiment here.

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I want to let you understand that

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by actually comparing yourself to others,

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which is actually a really good way
to make people understand this quickly,

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so I'm going to ask you to raise your hand
and tell us, and report to us,

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when you prefer to go to sleep
on a free day with no social obligations.

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Okay?

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So who goes to bed before midnight?

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Okay, look around and see
how many hands are up.

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How about midnight to 1:00?

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1:00 to 2:00?

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2:00 to 3:00?

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(Laughter)

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And after 3:00?

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There should be
some people here after 3:00.

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So you've just reconstructed
the chronotype distribution

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right here in this room.

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So now you see how it works.

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I recently came across a dramatic story
concerning chronotype.

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This is a picture of the famous
castle Neuschwanstein.

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It was built by Ludwig II,

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who became the king of Bavaria
in 1864 at the age of 18.

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Twenty-two years later
he was forcibly removed from the throne

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when he was pronounced insane

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by a board of psychiatrists
who had never examined him.

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Ludwig may have had some troubles
in the mental health department -

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I think no one can know
for sure; it's history.

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What we do know

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is that despite some considerable
popularity with his subjects,

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the government was deeply disturbed

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that through his palaces
and castles that he was building,

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he was pushing the state
to the brink of bankruptcy.

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They wanted him out.

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When I read a bit more
about the daily life of Ludwig,

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I became suspicious
of the diagnosis of mental illness.

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Certainly, he wasn't coping
very well in general,

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and he practised
some eccentric behaviours,

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but the most specific description
concerned the timing of his behaviour.

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He ordered breakfast
at 6:00 in the evening,

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lunch at midnight,

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and dinner in the early morning

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before retiring to bed
to sleep through the day.

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Very rarely one finds
a reversed chronotype like Ludwig had.

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He seems indeed to have been
most relaxed at Schloss Linderhof here,

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where he famously was riding around

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in his sleigh through the night
around the grounds.

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In contrast,

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his official duties would have taken place
during the daylight hours,

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which was unfortunately
during his night time.

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Think about how you would feel
if you had to negotiate anything

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in the middle of your night.

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Ludwig found himself negotiating
the independence of Bavaria

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in the middle of his night,
and he lost it.

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This misstep essentially
created the German Empire.

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(Laughter)

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I find myself wondering
if the chronotype of Ludwig II

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changed the history of the world.

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(Laughter)

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That's of course impossible to say,

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but extreme examples like this
are very useful for making a point,

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and the point here
is that like other living creatures,

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we have our own
individual circadian clock

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that in combination with our age
and our light exposure

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will make us a certain chronotype.

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Our clock directs
the timing of our behaviour.

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Our chronotype is so strong

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that it sometimes isolates us
like it did Ludwig II.

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It does this in small ways.

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When we want to be with someone
who has a different chronotype,

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maybe we feel like doing something
that they don't feel like doing.

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Our chronotype is so strong

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that sometimes we have to modify it
with an alarm clock,

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otherwise we might get in trouble
with work or school.

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Both of these scenarios are suboptimal.

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One of them results
in social relationships that suffer,

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and the other results in sleep deprivation

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as a result of the clash
of the biological and the social clock;

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this is a condition
we call social jet lag.

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And sleep deprivation is known
to lead to a variety of problems

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ranging from poor performance to illness.

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Is there anything, then, that you can do
to fine-tune your chronotype

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so that you can have more harmonious time
with family and friends,

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and you can lose less sleep to your work?

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Can you unshackle yourself
from your clock?

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The answer is: almost certainly.

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If you think back to what I said
regulates chronotype -

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that would be genes, light and age -

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two of those things
you can't change overnight.

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But light is something
that we can work with.

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It's easy to find, it's cheap
and it's not unpleasant.

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So the only problem is
that we don't have very good prescriptions

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for how light will interact
with your specific chronotype,

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because remember, you have
a unique collection of clock genes

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that we don't really understand yet,

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so we don't know
how they would react to light.

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There is a general rule
that most people, however,

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if they increase the light
they get in the morning

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00:16:28,926 --> 00:16:31,126
and decrease the light
they get in the evening,

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for instance, from computers
and televisions,

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that they will tend to move earlier
in their chronotype

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00:16:36,455 --> 00:16:39,147
and therefore have to use
their alarm clock less.

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Okay?

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Okay, 80 percent of you out there
are alarm clock users.

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This should remind you every single day
that you have a circadian clock.

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Your clock is showing itself
in your individual cells;

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it's showing itself in your family
and in your community.

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Despite the alarm clock,
which I hope you'll use now much less,

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the circadian clock is a wonderful
addition to our poor existence.

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It gives us more versions of ourselves.

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We are, after all, different people
morning and evening.

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Get to know you.

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And since I'm a scientist,
I have to acknowledge my dear colleagues,

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because no scientist stands up here alone
without a huge team behind them.

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Maria and David who are here today,

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00:17:33,639 --> 00:17:37,228
Till Roenneberg who's rewritten the book
on human behaviour,

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and many chronobiologists,

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and please do discover your chronotype
at this web address: [www.euclock.org]

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Thank you very much.

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(Applause)