Ekadashi is the 11th day of the moon cycle, both from the full moon and from the new moon.
The Padma Puraana, describes the following relevance of Ekadashi:
Jaimini Rishi, a renowned sage, once became inquisitive about the Ekadashi vow, so he inquired from the great sage Vyaasa about the same. Vyaasa said that initially, when the world was manifested, Lord Vishnu created a demonic creature (Paapa-Purusha) that was the embodiment of all types of sins. This was done in order to punish all beings who would choose the path of evil. Subsequently, He also created the Yamaloka - the cosmic penitentiary, so that anyone who sinned (with symptoms of Paapa-Purusha in him) would be sent there.
Once upon a visit to Yamaloka, Lord Vishnu noticed the miserable state of the living entities there undergoing "corrections", and had pity on them. So He created Ekadashi from His own being, and decided that anyone undertaking the Ekadashi vow would be cleansed of their sins and will not have to visit the cosmic penitentiary.
Becoming aware of this, the Paapa-Purusha became alarmed. He immediately went to Lord Vishnu and pleaded that because of these Ekadashis soon he may not have any sustenance. So the Supreme Personality of Godhead gave him a benediction that he may reside in the Beans, Grains and Cereals on the Ekadashi days. Thus he may affect anyone who consumes these on an Ekadashi. At this the Paapa-purusha became satisfied.
The Ekadashi day being such a wonderful day to cleanse one's offences is thus respectfully also celebrated as the day of Lord Hari (the Supreme Lord God).
Meanwhile according to modern science, it is known that the air pressure on our planet varies to extreme limits on both the new moon (Amavasya) and the full moon (Purnima) day. This is because of the orbital path combination of the sun, moon and earth.
This can be observed by the change in the nature of the tidal waves on the new moon and full moon days. The waves are very high and rough, but from the next day onwards, the waves become calm, an indication that the pressure has also receded.
Now, based on this fact, the significance of Ekadashi fasting can be explained in 2 ways:
1) According to science, it takes about 3-4 days for the food that we eat today to reach our brain. Now, if we eat light/fast on Ekadashi days, that intake will reach the brain correspondingly on the New moon/full moon day.
On both of these days, the earth pressure is at its maximum, thus leading to imbalance in everything, including ones thought process.
So, if the input to the brain is at a minimum, the chances of the brain indulging in any wayward activity due to the high pressure imbalance also becomes minimum.
2) Another explanation for the Ekadashi fasting is that compared to any other day of the moon cycle, the atmospheric pressure is the lowest on the Ekadashi days. Thus, this is the best time to fast and cleanse the bowel system. If we fast on any other day, the high pressure/strain may damage our system. Thus, it is advisable that after fasting on the Ekadashi, on the immediate next day (Dwadashi), we get up early and eat as soon as possible.
As per both the above theories, the fasting practice on the Ekadashi days have a strong scientific base. People who observe the fast are asked to stay away from all types of grains, and to have a light diet of nuts, milk, fruits, etc.
Fasting gives the system a rest. The physiological system may become overworked due to a little overeating or indiscrimination in diet. Thus the fortnightly Ekadashi fasting gives the system a chance to catch up. We know that the digestive system draws the blood circulation towards the digestive organs. Therefore blood circulation to the head is decreased once food is taken: so we feel sleepy. Thus observance of Ekadashis helps us recharge our brain and mind keeping us more alert, sharp, focused and more aware.
The fortnightly Ekadashi fasting accompanied with healthy eating improves insulin responsiveness, lowers blood cholesterol and prolongs life span. It helps improve the mental stability of people suffering from anxiety and depression. It also detoxifies the body, cleanses the blood and improves the functioning of kidneys and liver. It is amazing how the ancient Vedic Indians devised this method to keep ourselves fit and free from any negative influences!
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The following article that I found on the net might interest you too:
A man or a woman is 80 percent liquid and 20 percent solid like the earth, This is the basic biological condition which helps the understanding about the Ekadashi fast.
We are aware of high tide on new moon and full moon days and low tide on the seventh day of the lunar cycle. This is lunar attraction on the liquid of the earth. The earth and the moon attracts each other but on certain days the attraction is more because of their nearness. It is this attraction that causes tides in the ocean and it is certain that there will be biological high tide and low tide too, governed by the moon. The gravitational force of the moon exerts its influence on the water in the human body as it does in the case of the oceans of the planet.
Arnold Lieber, a Miami Florida psychiatrist, experimented and found that the biological tides effect our moods and behaviours also. From many of the psychiatric hospitals it has been reported that at full moon or new moon days mental patients' behaviour becomes increasingly disturbed and erratic. In an article, "Does the moon control your moods" Edgar Ziegler writes that the Phoenix Arizona fire department has found that it receives 25 to 30 more calls on nights of the full moon. There are instances, that on these tidal days when the moon comes closest to the earth, people having weak mind or a week body specially suffer more. I know many people with a weak bowel who always complain of the worst suffering on such days.
There are fantastic reports quoted in the article of Edgar Zieglar mentioned above. He writes, that Fergus Wood a scientist with the U.S. National Ocean Survey, reported several years ago, that two extreme high tides would occur, one after another, on January 8 and February 7 of 1974. Wood saw that on both occasions, the earth, sun and moon would be positioned along a nearly straight line called a Sygy and that on January 8 1974 the moon would also be very close to the Earth. This frequent combination of astronomic events causes ocean water to rise far above normal levels. On hearing this Arnold Lieber alerted the Miami police department, newspapers and psychiatric emergency rooms at Miami's Jackson Memorial Hospital predicting a general disturbance in human behaviour, during these days, And really, Miami's murder toll for the first of these weeks of the new year was two times higher than for all of January 1973. Also, there were repeated incidences of motiveless crimes. This was one such example to quote from on how human behaviour is affected by the lunar attraction on human personality.
Studies on the influence of celestial bodies upon ours have shown an imbalance of hormones and fluids can occur, and that these influences can play a greater part on our pathological diseases and psychological behaviour. The physiological glands and hormones concerned relate to the physico-psychic reasons for the various psychic expressions known as vrttis (propensities), e.g. fear, greed, hatred, passion and anger etc. There are 50 such expressions and if these hormones get imbalanced, the secretions become abnormal, i. e. hypo or hyper, resulting in abnormalities with these expressions (vrttis). The above experiment of Arnold Lieber was for a few of these vrttis only; but all such or many of these vrttis can get affected by lunar biological tides.
Not only that, the skin of our body is a semi-permeable membrane that permits movement of electromagnetic forces in both directions, maintaining a dynamic equilibrium. Arnold Lieber says that each nerve impulse generates its small aura of energy for the associated cells, like a miniature solar system, and has its own faint electromagnetic field. It is possible that the gross electromagnetic forces originating with celestial bodies can affect the balance of these microscopic cellular worlds. When there will be extreme tides, there is a chance of heavy bombardment by massive disturbances of grave and surrounding electromagnetic fields. This may cause effects to the nervous system and weaken the nerve fibre.
In Lieber's research on a number of manic depression patients, he found recurrences of symptoms of depression, restlessness, insomnia and rapid heartbeat during extreme tidal days. It is commonly found that a range of attacks are more on tidal days and the effect is from Ekadashi to New Moon or Full Moon days. Thus Ekadashi fast has varied effects on the body and the mind to counter negative effects. The first and foremost is keeping the balance of hormones and other secretions in relation to the glands and cells. As there will be no food and water in the stomach on these fasting' days, the linear attraction will not affect the intestine, kidney and liver etc. Further, the tendency of the functioning of most of the forces of the body will be centripetal, i. e. towards the inner side of the body. So, the internal gravitational pull will be more powerful to balance the external pull of the lunar gravitational force. This will help in controlled functioning of all the organs, chemical changes, growth of cells and other biological transmutations.
If functioning be normal and balanced, there will hardly be health problem and the longevity will get enhanced many folds. It is good to fast on both Ekadashi days and Full Moon and New Moon days. But at least the Ekadashi fast is a must for all person, male and female above the age of 12 years. As Ekadashi day falls in between the seventh and fifteenth day of the lunar cycle, i. e. the neap tide and high tide days, a day's fast helps balancing the effect of the lunar attraction. But, as mentioned above fasting can be done on all the four days for better results, that is: Ekadashi before New Moon, New Moon itself, Ekadashi before Full Moon, Full Moon itself. The chance of bombardment of celestial electro-magnetic forces on the tiny cells of our body too, will have less or no effect at all, thereby the equilibrium can be better maintained.
It has been experimentally seen that manic-depressive patients get cured by such systematic fasts and many abnormalities of the mind which mostly the people of the modern world suffer from to-day, will get reduced. Excessive sexual excitement, anger, irritation, fear, greed and passion etc. also gets sublimated by these fasts. Even hyper-tension can be controlled efficiently with the help of Ekadashi fast.
Mostly people fear that by fasting one will become weak. Such a fear is not genuine. A day's fast gives rest to the machinery of stomach and thereby helps it to function better in the future. Not only that the undigested food material gets proper opportunity for its further digestion, but also by fasting the digestion power increases, better assimilation of food takes place helping in more production of blood and other necessary chemicals, and thereby helping better health and better nutritional effects. It is both a good measure for would be stomach troubles and a cure for a number of ailments. It helps better functioning of the liver, pancreas, intestines and kidney etc., thereby reducing the chance of any ailment relating to these organs.
The fast on Ekadashi also helps the transmutation of chemicals in the body into subtler factors like ectoplasmic stuff of the mind (i.e. chitta or mindstuff) so that the ectoplasmic stuff associates with the still higher spheres. If the science of meditation is practised by the aspirant, fasting does not only mean not eating anything, it also means not drinking any thing in the stricter sense. The meaning of fast, i.e. 'upavasa' spiritually means staying with divine ideas, i. e. living closer to the Supreme Conscious Being. This can be done only by shunning the activities of the sensory and motor organs (indriyas) extroversally and dwelling instead on the various aspects (bhaava) of the Supreme Consciousness or the Supreme Lord. By not eating and drinking on Ekadashi days along with dwelling on the divine (bhagavata-bhaava), it will certainty help an aspirant to keep himself or herself perfect physiologically and psychologically and thereby help attain the ultimate spiritual purpose of life.