Three different origins can be identified:
1) Many of those who study trichology recognize a genetic heredity combined with the seasonal change of the hair that is characteristic in mammals;
2) Other scholars believe that it is due to the change in the relationship between the hours of light and dark that cause the hair loss by influencing the hormonal balance;
3) Finally, other trichologists provide an explanation more closely related to psycho-sociological factors of the phenomenon and associate the increase in hair loss with the return to polluted city life after summer vacations, characterized by stress, returning to work, frenetic rhythms of life and the presence of atmospheric pollution.

Furthermore, we can confirm that some scientific studies claim to have found the cause for seasonal hair loss. Considering that after 3 months any damage or benefit at the level of the scalp translates into a loss or improvement, the autumn hair loss would apparently refer to excessive exposure of the head to sun during the hot summer months (the rays of the sun cause free radicals to accumulate, and these damage the hair bulbs and are therefore responsible for the hair loss after approximately 3 months, thus in autumn); with regards to the spring loss the theory states that excessive exposure of the head to cold during the winter leads to vasoconstriction at the level of the scalp, thus damaging the hair bulbs and resulting in a loss after approximately 3 months, thus in the spring. Regardless of the cause of seasonal hair loss, its increase in the spring and autumn is considered to be a physiological phenomenon of natural “renewal”.

It is well known that human hair has a growth cycle that may last between two to six years. Hair, in fact, is renewed around twenty times in the arc of your entire life. This means that each hair falls out and regrows around twenty times before definitively dying following aging of the hair follicle. The spring and autumn periods do not do anything other than accelerate, due to changes in climate, the life cycle of hair and its physiological renewal. 
That said, a distinction must be made between seasonal hair loss and irregular or pathological hair loss. 

For the majority of people losing hair is a condition experienced with anxiety and depression. What happens in the spring and autumn periods is natural. In these seasons even for people with full heads of hair, hair loss can greatly exceed one hundred hairs per day. In these cases, only hair in the telogen phase falls out (in fact, we are talking about seasonal telogen effluvium), meaning that which has by now reached the end of its growth cycle. 
In fact, its follicles are already working to produce new hair. There are, however, also those clinical cases in which the natural increase in seasonal hair loss hides a real hair growth problem, thereby making it difficult to distinguish between normal and irregular hair loss. In fact, during the autumn period the renewal of psycho-physical tensions due to the beginning of the work year is reflected in hair health. If this stress is inserted into a situation of already weak hair, the risk is very high that a process of excessive and abnormal hair loss will be activated. 

There are also some signs that indicate that the hair loss has pathological characteristics, such as the presence of scalp itchiness and pain. It should be noted that often in the months in which hair loss increases in many people it tends to lead to an increase in pre-existing hair loss. 
In these situations it is always necessary to consult a trichologist who, by studying the individual case, will determine the causes responsible for the excessive hair loss and will establish a proper therapy to remedy it.