We have an average of 100,000 hairs on our scalp. Most hairs grow 1.25 centimeter per month for up to six years and then they fall out. About 90% of the hair on your scalp is growing at any one time and 10% at rest. After 2-3 months, the resting hair falls out and new hair starts to grow in its place. It is normal to shed some hair each day as part of this cycle. However, hormonal problems (thyroid, PCOD), nutritional deficiencies, auto immune conditions, complication to certain medications may affect the growth phase and cause abnormal hair loss.