The Hair serves as an outward form of recognition of Sikh men and women
Nature knows best what to discard or retain. Whatever is retained, is not without purpose. Hair is a gift from God, not a burden. Guru Gobind Singh Ji, in his infinite wisdom, commanded us to respect hair and refrain from tampering with it. This is the visible token of his affection for us, as well as our faith in him.
Kesh (wearing long hair) Since all Sikhs pledge to not cut their hair, Kesh reminds Sikhs of their equality, and removes reason for pride (Ahankar). In addition, all Sikh men are required to wear a turban or Dastar (its optional for women.
From the scientific view, keeping hair is practical because hair has many functions. It traps an insulating layer of still air just outside the skin, and thereby reduces loss of heat by radiation, hair absorbs harmful radiations from the sun, hair follicles can make androgenic hormones and Axillary hair provides larger surface area for evaporation of sweat.
Although Sikhs do not need scientific explanations to keep hair, it is important to note that scientific explanations do indeed exist. (Sikhism can be backed by Logic and can be supported by the Laws of Science).
The validity of the above Statement depends on "Who is talking to Whom".