Under normal conditions, scalp hairs live for about three years (the anagen, or growing, phase); they then enter the telogen, or resting, phase. During the three-month telogen period, the hair root shrivels up into a small white "club," then the hair falls out. It is, therefore, normal to lose about 100 hairs every day, more of them on days when shampooing loosens the hairs that are ready to fall out. The hairs are then replaced by natural regrowth.
Sometimes people worried about losing their hair start noticing hairs on their pillow or in the sink, not realizing that they've always been there. A close look at these will usually reveal the white "club" at the end, showing that these hairs were already dead. Normally, about 10% of scalp hairs are in the telogen phase.