Professor Terry Page holds one of the cockroaches that he used in his research. ((Daniel Dubois, Vanderbilt University))

Cockroaches, like humans, have dramatic daily variations in their ability to learn, say biologists at Vanderbilt University in Nashville.

More pointedly, cockroaches could use a strong cup of coffee in the morning, but appear to need no boost in the evening, according to their study, published this week in the Proceedings of the National Academy of Sciences.

"This is the first example of an insect whose ability to learn is controlled by its biological clock," Terry Page, professor of biological sciences, said in a release.

Most studies on learning and circadian rhythms have focused on mammals. For example, recent experiments with humans have found that people's ability to acquire new information is reduced when their biological clocks are disrupted.

In the current study, the researchers taught individual cockroaches to associate peppermint — a scent they normally find slightly distasteful — with sugar water, causing them to favour it over vanilla, a scent they like very much.

The researchers trained individual cockroaches at different times in the 24-hour day/night cycle and then tested them to see how long they remembered the association.

They found those trained in the evening retained the memory for several days. Those trained at night also had good retention. During the morning, however, when the cockroaches are least active, they were totally incapable of forming a new memory, although they could recall memories learned at other times.

"It is very surprising that the deficit in the morning is so profound," Page said. "An interesting question is why the animal would not want to learn at that particular time of day. We have no idea."

Learning more about how the biological clock modulates learning and memory may lead to a better understanding of what can influence them, said Page.