"How to Sync Your Central Circadian Clock to Your Peripheral Clocks"
One of the most important breakthroughs in recent years
has been the discovery of peripheral clocks.
We've known about the central clock, the so-called suprachiasmatic nucleus,
for decades. It sits in the middle of your brain right above
where your optic nerves cross, allowing it to respond to night and day.
Now we know there are semi-autonomous clocks in nearly every organ in our body.
Our heart runs on a clock, our lungs run on a clock, our kidneys run on a clock.
Up to 80% of the genes in our liver are expressed in a circadian rhythm.
Our entire digestive tract does too.
The rate at which our stomach empties, the secretion of digestive enzymes,
and the expression of transporters in our intestinal lining
for absorbing sugar and fat all cycle around the clock.
So too does the ability of our body fat to sop up extra calories.
The way we know these cycles are driven by local clocks, rather than
being controlled by our brain, is that you can take surgical biopsies of fat,
put them in a petri dish, and still watch them rhythm away.
All this clock talk is not just biological curiosity.
Our health may depend on keeping all these clocks in sync.
Think about it like a child playing on a swing.
Imagine you're pushing, but you become distracted by other goings-on
in the playground and stop paying attention to your timing.
So you forget to push, or push too early, push too late.
What happens? Out of sync, the swinging becomes erratic, slows, or even stops.
That is what happens when we travel across multiple time zones
or have to work the night shift.
The pusher in this case are the light cues falling on our eyes.
Our circadian rhythm is meant to get a bright light push every morning at dawn,
but if the sun rises at a different time or we're exposed to bright light
in the middle of the night, this can push our cycle out of sync
and leave us feeling out of sorts. That's an example of a mismatch
between the external environment and our central clock.
Problems can also arise from a misalignment between the central clock
in our brain and all the other organ clocks throughout our body.
An extreme illustration of this is a remarkable set of experiments
suggesting even our poop can get jetlag.
Our microbiome seems to have its own circadian rhythm.
Even though they're down where the sun don't shine, there's a daily oscillation
in both bacterial abundance and activity in our colon.
Interesting, but who cares? Check this out: if you put people
on a plane and fly them halfway around the world, then feed their poop to mice,
they grow fatter than mice fed preflight feces. Though it may have just been
bad airline food or something, the researchers suggest the fattening
flora was a consequence of circadian misalignment.
Indeed, several lines of evidence now implicates chronodisruption—
the state in which our central and peripheral clocks diverge out of sync—
as playing a role in conditions ranging from premature aging and cancer,
to mood disorders and obesity. Bright light exposure
is the synchronizing swing pusher for our central clock.
Well, what drives our internal organ clocks that aren't exposed to daylight?
Food intake. That's why the timing of our meals may be so important.
Removing all external timing cues by locking people away
under a constant dim light, researchers showed you could effectively decouple
central from peripheral rhythms just by shifting meal times.
They took blood draws every hour and even took biopsies of their fat
every six hours to demonstrate the resulting metabolic disarray.
Just as morning light can help sync your central clock in your brain,
morning meals can help sync your peripheral clocks
throughout the rest of your body. Breakfast skipping disrupts
the normal expression and rhythm of these clock genes themselves,
which coincides with the adverse metabolic effects.
Thankfully, they can be reversed.
Take a group of habitual breakfast skippers, and have them eat
three meals at 8 am, 1 pm, 6 pm, and their cholesterol and triglycerides
improves, compared to taking meals five hours later at 1 pm, 6 and 11 pm.
See, there's a circadian rhythm to cholesterol synthesis in the body too,
which is also strongly influenced by food intake, as evidenced by the 95% drop
in cholesterol production in response to a single day of fasting.
That's why just a few hours shift in meal timing can result
in a 20-point drop in LDL cholesterol, thanks to eating earlier meals.