The circadian rhythm in the body that quietly pulses inside us all, guiding our daily cycle from sleep to wakefulness and back to sleep again may be crucial to learning and memory, according to a study by Stanford researchers.
Biologist Norman Ruby, the lead author of the study, worked with Siberian hamsters and found that having a functioning circadian system is critical to the their ability to remember what they have learned.
Without the system, Ruby said, "They can't remember anything."
The findings have implications for diseases that include problems with learning or memory deficits, such as Down syndrome or Alzheimer's disease.
During the study, the researchers knocked the hamsters' circadian systems out of commission using a new non-invasive technique they developed involving manipulating the hamsters' exposure to light.
The hamsters were first exposed to two hours of bright light late at night. Then the next day the researchers delayed the usual light-dark cycle by three hours.
After the treatment, the normal light-dark cycle is resumed, but that one-time treatment is enough to wipe out their circadian system.
To assess the effect of the treatment, Ruby's team conducted a standard test called a novel object recognition task that takes advantage of animals' innate tendency to explore their environment. Using a box roughly 2 feet square, the researchers put two identical objects in adjacent corners, such as two saltshakers or two shot glasses.
The hamster is then placed in the box, on the opposite side from the objects. As it explores the box and the objects, the hamster spends approximately equal amounts of time on each of the two identical objects.
After 5 minutes, the hamster is removed from the box, and one of the objects is replaced with a new, different object. After a span of time-in Ruby's study, the time was varied between 20 minutes and an hour-the hamster is put back in the box.
"A normal animal will spend time with both objects, but it will spend easily twice as much time with the new one. It understands that it has seen the other one before," Ruby said.
However, when a hamster that lacks circadian rhythms is put back in the box, it's as if it is a whole new world for the hamster. Whether the hamster is out of the box for an hour or as short a time as 20 minutes, it spends the same amount of time with each object, Ruby said.
"What that means is they don't remember the object that was in there before," he said.
The finding is even more striking when you consider that when a hamster loses its circadian system, it gets even more sleep than usual.
"What our data are showing is that these animals still performed terribly on a simple learning task, even though they're getting loads of sleep. What this says is that the circadian system really is necessary for something that is deeply important: learning," Ruby said.
The work is described in a paper published Oct. 1 online in the early edition of the Proceedings of the National Academy of Sciences.