There are many factors associated with having lighter periods. Lighter periods or irregular menstrual cycle can be caused when a woman does not release an egg or does not ovulate. Other reasons for having lighter periods can be age factor, being underweight or overweight, pregnancy, overstress and several medical conditions such as PCOD or thyroid related problems.
Light periods can mean much different from time to time. At times, it can indicate early pregnancy and then in some case it can mean premenstrual syndrome. So, you need to consult your gynecologist for proper diagnosis, advice, and treatment of the same.