The reasons why hair becomes oily after a day of washing include endocrine, bad habits and impaired scalp barrier function. 1. Endocrine factors: If the endocrine disorders, there may be an increase in the level of androgens, which will cause a large secretion of sebum from the scalp, perhaps triggering the hair to become oily. 2. Bad habits: If you stay up late for a long time, or consume foods that are high in sugar and oil, it may trigger the hair to become oily after a day of washing. 3. Damaged scalp barrier function: If you over-clean or wash your hair frequently in your daily life, you may damage the scalp barrier function, and the symptom of oily hair after a day of washing may occur. There is no single reason for hair to become oily after a day’s washing. It is recommended to adopt the correct concept of washing and care, avoid over-cleaning, and have a regular rest and relaxation routine, so as to improve the situation of hair becoming oily easily. If you still can’t get relief, we suggest you go to the hospital and get a clear picture of the cause of the problem.