Philosophy behind it: Most devout Indians fast regularly or on special occasions like festivals. On such days they do not eat at all, eat once or make do with fruits or a special diet of simple food. Fasting in Sanskrit is called upavaasa. Upa means “near” + vaasa means “to stay”; Upavaasa therefore means staying near (the Lord), meaning the attainment of close mental proximity with the Lord. Then what has upavaasa to do with food? A lot of our time and energy is spent in procuring food items, preparing, cooking, eating and digesting food. Certain food types make our minds dull and agitated. Hence on certain days man decides to save time and conserve his energy by eating either simple, light food or totally abstaining from eating so that his mind becomes alert and pure. The mind, otherwise pre-occupied by the thought of food, now entertains noble thoughts and stays with the Lord. Since it is a self-imposed form of discipline it is usually adhered to with joy Also every system needs a break and an overhaul to work at its best. Rest and a change of diet during fasting is very good for the digestive system and the entire body. The more you indulge the senses, the more they make their demands. Fasting helps us to cultivate control over our senses, sublimate our desires and guide our minds to be poised and at peace. Fasting should not make us weak, irritable or create an urge to indulge later. This happens when there is no noble goal behind fasting. The Bhagavad-Gita urges us to eat appropriately - neither too less nor too much - yukta- aahaara and to eat simple, pure and healthy food (a saatvik diet) even when not fasting. The Science behind it: The systems propounded in the Vedas and Shastras coordinate human's biological rhythms with the cycles of nature. One of nature's most fundamental rhythms can be observed in the phases of the moon. According to scientific research, it is known that the air pressure on the earth 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. These phases must therefore affect the human body, considering it is approximately 70% water. The systems of fasting are based on the different stages of the moon's waxing and waning. The cycles of the moon influence women in particular through the menstrual cycle, so fasting is practiced more extensively by women. Thus these systems were formulated and recorded to enable all people to go beyond individual consciousness and experience the real essence of life. Fasting is systematically advocated in the scriptures in order to align the physical and subtle bodies with the whole cosmos, enabling sustained equilibrium in every aspect of life.