Menstrual cycle The first period usually begins between twelve and fifteen years of age, a point in time known as menarche. They may occasionally start as early as eight, and this onset may still be normal. The average age of the first period is generally later in the developing world and earlier in developed world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 35 days in adults (an average of 28 days). Menstruation stops occurring after menopause which usually occurs
Human sexual response cycle Human sexual response cycle The human sexual response cycle is a four-stage model of physiological responses to sexual stimulation, which, in order of their occurrence, are the excitement phase, plateau phase, orgasmic phase and resolution phase. This physiological response model was first formulated by William H. Masters and Virginia E. Johnson, in their 1966 book "Human Sexual Response". Since then, other human sexual response models have been formulated. The excitement phase (also known as the arousal phase or initial excitement phase) is the first stage of the human sexual response cycle, which occurs as a result of physical or mental