Research Links Circadian Rhythms to Memory
by Michelle Meyer and Nishma Sachedina
Many organisms have circadian rhythyms, internal clocks that are closely linked to the light-dark cycle of a day. Generally, if an organismÕs internal clock is unsynchronized, it continues to live a normal life with no serious physical consequences. If these clocks seem so inconsequential, why has Mother Nature preserved them throughout evolutionary history? Recent research by Dr. Norman Ruby, a senior research scientist in StanfordÕs Department of Biology, suggests that circadian rhythms are closely linked to the ability to learn new tasks.
Disrupting Learning
Ruby studied the relationship between circadian rhythms and learning and memory by conducting experiments in the lab of Dr. H. Craig Heller, Professor of Biological Sciences at Stanford. During the Ònovel object taskÓ experiment, researchers placed a Siberian hamster inside a box with two identical objectsÑin this case, shot glasses Ð and then recorded the time the hamster spent investigating each object. As expected, in this first case with two novel objects, the hamster spent equal time examining both. One hour later, researchers placed the hamsters back in the box with two objects, one of the old objects and one new, different object. The hamster spent twice as much time with this Ònovel object,Ó suggesting that the hamsterÕs hippocampus stored the memory of the previous experience with the two identical shot glasses.
When this experiment was repeated using arrhythmic hamsters whose internal clocks were altered by light-cycle manipulation, the animals displayed no memory of the old object after an hour and spent an equal amount of time with the old and new objects. However, when the second trial followed the first trial after 20 minutes rather than an hour, these arrhythmic hamsters showed some signs of memory and spent more time examining the novel object than the old object. These findings suggest that the memory of the two initial shot glasses was created but not retained in the hamsters with irregular biological clocks.
Daily Memory Cycles
Another intriguing study shows that learning a task is affected by temporal factors and that memory recall is best at 24-hour intervals. Forced to learn tasks at different times during the day, ratsÕ memories were tested at varying intervals after the tasks. Researchers found that the ratsÕ recall ability was best 24 hours after the initial task, indicating a correlation between circadian rhythm and memory.
Interestingly, according to Ruby, ÒHumans have optimal cognitive performance at ten in the morning--give or take a little,Ó further supporting the hypothesis that circadian rhythms influence the brainÕs ability to think and remember. As Ruby notes, these recent studies indicate that Òcircadian rhythms are clearly involved in learning and memory, though the how and why behind this link are currently unclear." Future Research
Ruby and other researchers in the Heller lab plan to study these mechanisms by conducting more behavioral research coupled with biochemical analyses. They hope to address why a loss in circadian rhythm leads to a loss in memory. In the near future, arrhythmic hamsters will be participating in a Òhole boardÓ memory test to determine if they can remember which nooks contained food and which were empty. This and future studies may reveal the reasons for circadian rhythms and lead to applications for preventing and treating memory loss.