We see hair fall every day. But have we lost the hair, never to regrow? On an average day, it is common to shed around 60-100 hairs. Over the scalp the visible hair count will remain roughly the same. With around 80-90% of the total hair count. This is due to the growth phases of the hair follicles under the skin. Reproducing new hairs to replace the old.
Certain conditions can upset this cycle of growth. Cause an increase in hair shedding, resulting in the in-balance of hairs lost and hairs growing. Common causes for this can be stress. Others, ill-health, poor diet and some medications. In some cases, the effects can occur rapidly and often for a relatively short period of time. But none the less can be rather traumatic to see more and more hair falling.
This condition is quite common and known as Telogen Effluvium. The shedding hair generally being in the telogen stage, or the resting stage. They would have fallen out eventually but are shed earlier than expected.
As said, a large percentage of cases the effects are short-lived and over time the hair growth cycle can return to normal with little to no visible signs that the condition occurred; chronic cases though can lead to visible thinning, normally in a diffused pattern across the top of the head, often leaving a hairline intact with thinning behind. Very severe cases can also affect the entire scalp hair, so around the sides as well.
In these cases, the problem may be not simple stress, ill health etc but a systemic problem or due to the use of medications that may have the side-effect of hair loss. If systemic a blood test can identify possibly an iron deficiency; if due to medications you should consult your doctor and possibly change medications if possible.