From the
article:
"In the current study, the researchers taught individual cockroaches to associate peppermint — a scent that they normally find slightly distasteful — with sugar water, causing them to favor it over vanilla, a scent they find universally appealing.
"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 that the individuals trained during 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,' says Page. 'An interesting question is why the animal would not want to learn at that particular time of day. We have no idea.'"