The Circadian Rhythm

Behavior is highly related to genes.  The most effective way to find the genes controlling behavior is to study the most instinctive and basic behaviors, because those genes are the most conserved, meaning the gene is present not only in every human, but also in most other species.

 There are many things we do on a daily basis that we take for granted.  We wake up and go to sleep on a twenty-four hour cycle, most of us at roughly the same time of day, everyday.  We may not realize this is actually one of our instinctive behaviors until we travel to different time zones.  Anyone who has traveled to a different time zone has experienced jet-lag, which can cause you to feel wide awake in the middle of the night, hungry at odd hours of the day.  This is actually caused by our internal clock - the circadian rhythm.  (See figure below for human circadian rhythm)

The circadian rhythm has been widely observed in plants, animals, and even fungi.  The circadian rhythm is basically the internal clock that dictates cyclic biological processes, such as sleep and hunger.  In order to better understand circadian rhythm, scientists started their studies on an animal with fewer genes, the fruit fly. 

Scientists were able to identify the key gene that dictates circadian rhythm through creating a fly that behaved oddly (i.e. it was active and slept during odd hours).  They did so by creating random mutations in almost every possible gene and observing the resultant behavior.  Eventually, they identified a gene they named “Period”, or “per”.  Among the fruit flies, scientists found four different types of the per gene: per, per⁰, per^s, and per^L.  The flies that contained the per gene had a normal activity cycle, a twenty-four hour time period; the flies that contained the per⁰ gene had a random activity cycle, a time period that could not be determined; the flies that contained the per^s gene had a shorter activity cycle, a nineteen-hour time period; the flies that contained the per^L had a longer activity cycle, a twenty-nine hour time period.  [Konopka & Benzer, 1971]

After the discovery of the per gene, scientists wondered if the gene exists in mice, which are mammalian and nocturnal (active at night), and if a mutation in the per gene has an effect on the mice’s circadian rhythm.  The answer is “yes”.  However, mammals are more complex than fruit flies.  Scientists not only found more than one type of “per”, but they also found other genes that affect the circadian rhythm. 

The discovery of the per gene has not only shed light on our understanding of our sleeping behavior, but also helped us to find the cause of some sleeping disorders, such as the “Familial Advanced Sleep Phase Syndrome (FASPS)”, found to be due to a mutation in the human “Period2 (Per2)” gene.  It is interesting that something which comes so naturally to us is actually controlled by an intricate genetic network.