Baldness is the state of lacking hair where it usually would grow, especially on the head. The most common form of baldness is a progressive hair-thinning condition that occurs in adult humans and many other ape species. Stress factors such as a regular lack of sufficient sleep may be able to accelerate the onset of hair loss in genetically prone individuals. Male pattern baldness is thought to occur in varying forms in about 50% of adult males. It is characterized by hair receding from the lateral sides of the forehead, known as "receding hairline" or "receding brow." An additional bald patch may develop on top (vertex). The trigger for this type of baldness, which is also known as androgenic alopecia, is currently believed to be 5-alpha reductase, an enzyme that converts the hormone testosterone into dihydrotestosterone (DHT), which inhibits hair growth. Onset of hair loss sometimes begins as early as end of puberty, and is mostly genetically determined. Male pattern baldness is classified on the Hamilton-Norwood scale I-VIII. Female pattern baldness, in which the midline parting of the hair appears broadened, is less common. It is believed to result from a decrease in estrogen, a hormone that normally counteracts the balding effect of testosterone, which normally occurs in women's blood. Female pattern baldness is being classified on the Ludwig scale I-III. There are several other kinds of baldness. Traction alopecia is commonly found in women with ponytails or cornrows that pull on their hair with excessive force. Traumas such as chemotherapy, childbirth, major surgery and severe stress may cause a hair loss condition known as telogen effluvium. Some mycotic infections can cause massive hair loss.