A penis that is hard when lying down and soft when standing up is an erectile dysfunction, which is related to factors such as mental stress, age factors, chronic diseases, and poor lifestyles. 1. Mental stress: for example, having a thought burden on sexual life, having unfavorable sexual contact, emotional factors, etc., may cause a decline in sexual function. 2. Age factor: with age, the level of androgen in the body may decline, which may lead to a decline in sexual function. 3. Chronic diseases: for example, chronic cardiovascular disease, diabetes, hypertension, high blood pressure, high blood fat, obesity, prostate disease, etc., may also lead to a decline in sexual function. 4. Poor lifestyle: such as overwork, lack of sleep, smoking, alcoholism, irregular diet, may also lead to decreased sexual function. There may be other reasons for the decline in sexual function, you need to go to the hospital in time for examination, to clarify the cause of the disease and follow the doctor’s instructions for treatment.