MPR: Working Nights

Working Nights
Part 2

In humans, the biological clock is a cluster of thousands of nerve cells that dwell near the base of the brain. From an evolutionary point of view, that's the oldest, most primitive neighborhood in a mammal's brain. The clock generates "circadian rhythms," a Latin word that means "about a day" because the cycles are roughly 24 hours long. The circadian timer makes you the most sleepy in the morning and the most alert in the evening.

SOUND: car door slamming
SOUND: street atmosphere
SOUND: French song on the radio, low and panned left

To demonstrate how it works, let's imagine that your body is an automobile. For the heck of it, let's make it an odd little French number with a temperamental engine. Every morning, a driver shows up to guide your body on a trip from sunup to sundown.

Woman in French, slightly off-mike: Bonjour, regarde ma petite Deux Cheveaux. Elle est magnifique, n'est pas?

The driver is your circadian pacemaker. Because your body's been parked all night, the engine is cold and sluggish first thing in the morning. It generally takes awhile to wake up.

SOUND: engine turning over, struggling to catch

Now, let's say that every morning a trailer is hitched to this automobile.

SOUND: clanking of trailer being attached
Woman in English: Hey, watch the paint job!
Dale Edgar: The trailer represents the body's accumulated need for sleep as the day wears on.

That's Dale Edgar of Stanford University's Sleep Research Center.

Edgar: After a good night's rest, the trailer should be empty. As the day wears on, the trailer gets loaded with an increasing weight of sleepiness. That sleep load keeps building, threatening to slow your body down.
SOUND: car engine beings to rev higher

The circadian driver tries to keep the body's speed steady in its journey from morning to night.

Edgar: By evening, the trailer is getting so heavy with accumulated fatigue that the circadian pacemaker must REALLY hit the gas to keep you awake.

As the trailer gets heavier, the pacemaker pours on more gas in the form of brain signals that promote wakefulness. Your alertness lags a bit after lunch, but then steadily rises through the late afternoon. And like a hot engine, your body's core temperature also rises. Then, after about 10 PM or so, the driver lets up on the throttle and shuts down for the night.

Edgar: By evening the trailer is getting so heavy with accumulated fatigue that the circadian pacemaker must REALLY hit the gas to keep you awake.
Sound: car engine really revving high now

Then, after about ten PM or so, the driver lets up on the throttle and the engine shuts down for the night.

SOUND: car dies; sound of it rolling to a stop
Edgar: All that sleep drags you down very quickly. That's the sudden feeling you can get in the late evening that you're really worn out and ready to snooze.
SOUND: clanking of trailer being unloaded

Scientists believe that during sleep, the body's fatigue is unloaded from the trailer, the car is refueled, and when the circadian driver is ready the next morning, the journey starts all over again.

Edgar: But it's important to note that it's not just the number of hours of sleep that matter, but the quality of sleep. If you can sleep through the night with fewer brief awakenings, then you unload that sleep more thoroughly. If you get a bad night's sleep or not enough hours, then the trailer is only half empty when you get going the next day. That sleep burden, or what we call the sleep debt, can accumulate day after day, making you more tired and less alert.
Music: drum theme rolls again

Now remember that this car metaphor is a generalization. Different people have different circadian drivers within them. Some feel most alert in the morning, others at night. And no one knows exactly why.

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