Eyelashes have a much shorter growth cycle than the hair on your head.

The human eyelash follows a three-phase life cycle consisting of the anagen, catagen, and telogen phases. Unlike scalp hair, which has an anagen (growth) phase lasting between two and seven years, the anagen phase for an eyelash typically lasts only 30 to 45 days. This limited growth window ensures that lashes do not grow indefinitely like the hair on our heads.After the growth phase, the lash enters the catagen phase, which is a transition period lasting two to three weeks where the follicle shrinks. Finally, the telogen or resting phase lasts for about four to nine months before the lash naturally falls out. At any given time, roughly 90% of scalp hairs are in the growth phase, while only about 15% of eyelashes are growing simultaneously.According to research published in the Journal of Optometry, the primary evolutionary purpose of the eyelash is to redirect airflow and moisture away from the cornea. By maintaining a length roughly one-third the width of the eye, lashes create a barrier that reduces evaporation of the tear film by up to 50%. This specific length is maintained precisely because of the short hair follicle cycle.If eyelashes grew at the same rate as scalp hair, they would eventually become heavy and obstruct the field of vision, posing a significant survival disadvantage. The average person has between 90 and 150 lashes on the upper eyelid and 70 to 80 on the lower eyelid. This biological regulation ensures the eyes remain protected from debris and sweat while maintaining optimal visual clarity.