Tired, hungry, sad? Maybe your circadian rhythms are having their way with you. Again.
“What are these rhythms good for? That is the basic question everyone is asking,” says neuroscientist Colleen McClung, of the University of Texas.
Circadian rhythms, which take place over the course of a day, are responsible for changes both mental and physical, such as alertness and hormone levels. They are at the heart of a rapidly expanding scientific field, the study of a region of the brain called the suprachiasmatic nucleus (SCN).
Although this collection of about 20,000 cells near the base of the brain was discovered three decades ago, researchers are in hot pursuit today of the secrets behind the mechanism that plays a key role in jet lag, depression and hunger, not to mention sleep.
Earlier this month, McClung joined several hundred circadian rhythm researchers at Cold Spring Harbor (N.Y.) Lab’s “Clocks & Rhythms” symposium.
“On so many levels, this is a very exciting area for biologists,” says Harvard University’s Charles Weitz. “There are so many things we know nothing about—we don’t even really know why animals sleep.”
Folks with a healthy SCN, even if left in a dark cave, typically conform to a slightly longer than 24-hour day, sleeping and waking in regular patterns, notes chronobiologist Claude Gronfier, of the French National Institute for Health and Medical Research. Damage to the SCN starts people “free-running,” waking and sleeping with no set routine.
Every day, the brain takes in light from the eyes to the SCN, resetting the daily clock.
Animals, plants and microbes display circadian rhythms, Weitz says. “This is a very deep part of biology, with genes driving the process.” Those genes became obvious to researchers working in mice and fruit flies in the last decade, he adds, opening a “golden age” of circadian research.
Help for the sleep-deprived
For folks just looking to get a better night’s sleep, circadian rhythm studies offer a leg up. Last month, Gronfier and colleagues unveiled a light pulse trick for fooling the circadian clock. Working with 12 volunteers, the researchers found they could shift the natural rhythm’s duration one hour backward or forward with exposure to bright pulses of light.
Researchers are also investigating the effects of circadian rhythms on obesity, suspecting that disruptions in sleep and eating routines throw off the body’s clock, setting the stage for weight and metabolic disruption.
An out-of-whack circadian system seems to be at fault for jet lag, Weitz notes. But the culprit isn’t the SCN, which adapts to sunlight cues in the new time zone, but rather “peripheral” body clocks located in virtually every organ.
The SCN cues these organ clocks to start ticking. But their activity seems more tied to food intake, and they respond more slowly to changes in meal times. Jet lag means your muscles and organs are expending energy at a time their internal clocks are trying to save it. So, you feel crummy.
Triggering serious disease
Circadian rhythm disorders can trigger serious problems, notably depression. Seasonal affective disorder is a winter depression tied to a lack of the sunlight cues that trigger the SCN into proper rhythm.
Also, there are indications that bipolar disorder also involves circadian problems, McClung says. This disorder, which causes unusual shifts in mood and energy, afflicts some 5.7 million people nationwide, according to the National Institute of Mental Health. “People might sleep all the time or not sleep at all,” says McClung. Body temperatures and hormone levels fluctuate, all pointing to a body clock with its springs missing.
