in "The Tree of Yoga"

"Anything done spasmodically has only a spasmodic effect. If you only practice spasmodically, you cannot expect to mantain the sensitivity of intelligence nor the maturity in the effort required to progress towards the ultimate goal. You must cultivate a certain discipline so that you can mantain that creative sensitivity. Instead of working as and when you feel, it is better to work regularly every day in order to mantain the quality of the effects. If your practice is irregular, there will still be more effects, but they will not be of the same quality." BKS Iyengar