Thread by @DrMikeFarquhar

The joys of the Internet!

Great to be watching @harvardmed sleep medicine legend Chuck Czeisler delivering a celebratory lecture on “A Brief Journey Through Circadian Time” @BrighamWomens, from the comfort of my office 💤 💡

Prof Czeisler talking us through the history of human circadian research, starting with some of the early experiments confirming the tendency of humans to “free-run” if deprived of external cues about day/night, light/dark

The factors regulating sleep are complex, and we *still* don’t understand them all.

Prof Czeisler demonstrating how this early data was already hinting at some of the multiple oscillating processes underlying our circadian rhythms

“We sat on this data for a decade it seemed so unexpected” ... Prof Czeisler describing the research that confirmed the evening “second wind” phenomenon and its associated wake maintenance zone

(Why it’s so difficult to fall asleep in the hours before your habitual sleep time)

“Even when you’re crazily sleep deprived -and we did that- people *still* feel alert in their circadian wake maintenance zone”

... which, Prof C points out, contributes to perpetuating chronic sleep deprivation, because people are tricked into feeling less sleepy than they are

This is also one of the factors which affects people’s perceptions of how safe they are to drive after a night-shift

(They did research with EEGs applied to drivers!)

So, understanding circadian physiology better makes sense for lots of reasons

(This I think is going to be one of the key areas of change in 21st century medicine ... though I may be biased)

Prof C now describing the early research looking at timing of light pulses to attempt to shift timing of our circadian clocks, and how same stimulus can have different effects at different times: idea of phase response curve developing

... and how this then moved to work tracking temperature and melatonin profiles in relation to this

“At the time humans were felt to be unique, in that they weren’t responsive to light cues, and social cues overrode light exposure”

... this is why history of medicine is so important, being aware of how much paradigms can shift even within our own lifetimes!

First circadian work all in men: all of Prof C’s initial applications to study circadian phase in women were rejected: “you can’t study circadian phase in women, they’ve got menstrual cycles as well”

... “it never made any sense to me!”

(When legislation passed by President Clinton changed this to require research to be done in men and women, a difference in circadian rhythms was demonstrated ...)

Prof C describing the work that led to our understanding of how both timing, dose strength, and frequency of stimuli to attempt to shift circadian phase developed

(it’s fascinating hearing the story of how our understanding evolved versus our current knowledge!)

We’ve moved on from space missions to Mars, to the research which started to demonstrate that light could still affect circadian rhythm, even in blind people (revolutionary at the time ... there’s more to eyes than rods and cones :) )

(also, sleep medicine is COOL!) 😁

(This is the work that led to the discovery of the photosensitive retinal ganglion cells which directly influence circadian rhythm by a separate neural pathway to the one that is involved in vision, and which are particularly sensitive to blue light)

Light exposure at particular times - even when it’s very short - can have a big effect on circadian phase. Brief pulses of light at night (bright light for a toilet trip, quick look at your social media notifications) can have disproportionate impact on sleep/wake timing

(One reason why sleep doctors will always tell you to keep your screen devices out of your bedroom, and avoid in the late evening pre-bed ... this can contribute to pushing your wake maintenance zone later in the evening, making sleep harder to initiate)

90 minutes listening to Prof C has flown by, and he’s still got lots more to say!

Finishing off with a look at the changing patterns of circadian phase in response to how we live today, and differentiating impact of circadian disruption from chronic sleep deprivation

Final point from Prof C looks at work to reduce shift length for night-working doctors which - once workload is also accounted for - demonstrates a 46% reduction in medical errors when US residents move to shorter (16 hour not 30 hour...) shifts.

(They initially had a seemingly paradoxical result where shorter shifts seemed to result in increased errors, until they realised it was in units where residents were effectively doing the same amount of work in shorter periods of time)

... I could have spent all day listening to Prof Czeisler talk

Great overview of his career’s work (to date!) in circadian medicine and still SO much for us to understand and apply to routine healthcare

Isn’t sleep medicine brilliant? 😁💤

(Thanks @harvardmed @BrighamWomens!)