Stress directly affects sexual fitness in several ways. It can potentially interfere with male erections and female sexual arousal, cause a drop in libido, testosterone levels and reduce fertility.
Acute stress restricts blood flow to the genitals, which hinders physiological arousal. A stressful event - whether in the form of a car accident, a missed meeting, conflict in the workplace, or unhappy news - invokes the fight-or-flight response. When this happens, your nervous system is activated and your adrenal glands release the stress hormone adrenaline into your body. Adrenaline causes your heart rate to rise and your blood vessels to constrict so that blood is delivered to where it's needed - the muscles, and not the genitals. This response can save your life in case of crisis by enabling you to move more quickly and powerfully, but it inhibits male erections and female sexual arousal. During and immediately after a jolting, stressful event, adrenaline actually prevents men from being able to achieve or sustain erections.
Over time, chronic stress may also interfere with male erections and female sexual arousal by causing cholesterol levels to rise, exacerbating arteriosclerosis, or hardening of the arteries. Anything that harms the cardiovascular system is bad news for sexual fitness, because narrowed arteries mean restricted blood flow through the tiny arteries of the genitals. Lack of testosterone due to chronic stress may also result in impaired fertility in men. In a study of men at a fertility clinic, sperm concentration and total number of active sperm decreased significantly when self-reported stress increased.