According to Hindu epic, Padma Purana, that describes the mythological relevance of Ekadasi.
On a visit to Yamlok(the palace of Yam), Lord Vishnu noticed the sufferings of the people living there and had sympathy for them. He created goddess Ekadasi from his female powerand decided that anyone undertaking the Ekadasi fast would be absolved of his sins.
Papa-Purush became alarmed. He immediately visited Vishnu Dam and met Lord Vishnu and pleaded that on Ekadasi day, he had no place to hide. So Vishnu told him that the rice would provide a place to hide on Ekadasi.
According to scientific research, the actual reason for this is as follows:
It is known fact that the air pressure varies to extreme limits on our planet Earth on the new moon (Amavasya) and the full moon (Purnima) day. This is because, on these days sun, moon and earth come in the straight line.
During these days, there are high tidal waves on the new moon and full moon days due to the gravitational pull of moon but from the next day onwards, the tidal waves become calm that gives an indication that the pressure has also receded.
According to scientific research, it takes about 3-4 days for the food you eat today to reach your brain (in the form of blood. Now, if you eat light or fast on Ekadasi days. It means that just avoid cereals especially the rice that is considered as the rich source of the carbohydrate. However, food that will reach the brain in the form of the blood correspondingly reaches on the New moon or full moon day.
On new/full moon days, air pressure is high thus leading to an imbalance in everything that include our thought process.
Thus, if the input to the brain is at a minimum, the chances of the brain indulging in any unnecessary activities due to the high-pressure imbalance also becomes minimum.