Estrogen tends to increase the anti-respiratory pituitary hormone prolactin. In mammals, prolactin has been called the “molting hormone” for its well-known ability to shed old feathers, hair, or skin, to make way for a new growth. Like other mammals, humans appear to be subject to seasonal differences in hair growth, with evidence that both the duration of hair growth and daily growth rates are greater in summer than winter. In fact, an old paper from 1947 explained that one constant feature of hair growth in man was that “summer” hair was more long-lived than “winter” hair. More recently, it was proposed that the hair follicle functioned as a “specialized UV receptor” responding to the nuances of seasonal light input. The marked seasonal effect of hair growth is so remarkable that it has been suggested that any new drug or treatment for baldness should be studied for at least a year to separate any effects from normal seasonal variations.