In the duration of the twenty-four hour day, there should be an overall balance between the two flows of ida and pingala. That is, the flow through the ida should predominate for about twelve hours, and the pingala also for about twelve hours. Remember, this does not mean that one flow will predominate for a single stretch of twelve hours, but that the total time is twelve hours, composed of smaller durations. If these intervals are equal, then it means that there is a balance between thought and action, between extroversion and introversion.
During daylight hours, the pingala generally predominates. This certainly applies to people who have to use a lot of physical exertion in their occupation; but it also applies to people who do a lot of mental work, for thought has to be translated into external action. Of course, ida will also flow, but it will tend to be subordinate to the pingala flow. Work generally implies dealing with other people and outside events; pingala will therefore tend to predominate during the daytime.
In order to maintain balance of the ida and the pingala, ida will tend to predominate during sleep at night, which counteracts the predominant pingala flow during the daytime.
It is also worthwhile pointing out one reason why people who practise yoga tend to need less sleep. Whereas the average person is totally absorbed in his or her work, the sincere yoga practitioner tends to do everything with a greater sense of detachment; that is, he maintains an attitude of introversion even during strenuous work, maintaining a greater balance between introversion and extroversion throughout the day. This implies greater balance between ida and pingala. Other people need more sleep to compensate for the greater flow of pingala during the daytime. Of course, even great yogis tend to need some sleep because there is always some imbalance. In addition, there are many other factors (moon, sun, climate, etc.) which affect the ida and pingala flows.
If one is excessively worried about work (or any other external preoccupations), and if one is pushing the body beyond its limits, then pingala will tend to predominantly flow day and night. If this imbalance continues over a long period of time then the entire physical and pranic body will become disharmonized.
The result will be illness. This is the situation of large numbers of people in the modern competitive world; they are too active and tense. This is one of the reasons for the prevalence of diseases such as cancer.
On the other hand, a person who broods a great deal, who continually dwells on his problems and who does little work or has little outside interests will have a predominance of ida. He also will suffer eventually, whether from lack of physical exercise, or mental problems such as neurosis.
Thus balance between the ida and pingala is essential. There must be equilibrium between introversion and extroversion. When this is not the case, the human system eventually takes steps to compensate this imbalance. This is manifested in illness, fatigue, depression or whatever. This is a natural law; if you break this law of the body then you must pay the penalty. If you feel tired, then rest; if you feel active then work or play hard. Try to follow the natural rhythm of your body and mind. It is when the body is pushed too hard, because of ambition, fears, desires or whatever, that imbalance occurs. Listen to your body and mind and follow its laws. This is not easy, but through the practise of yoga it becomes progressively easier.
There are a large number of factors which influence the physical, pranic and mental aspects of man, both in the internal and external environment. Any generality that is made about these factors has a tendency to be inadequate. To attribute the predominance of the ida flow or the pingala flow to any one specific cause is also totally inadequate. One has to understand the totality of a situation to obtain a true picture. This requires a high level of intuitive understanding and is the reason why so few people follow the path of swara yoga. We have given no specific details on the factors that determine the dominance of flow in the ida and pingala; we wish merely to indicate the general principles involved and their basic meaning and implications.