Our scalp sebaceous glands cause nighttime greasiness. Sebum from these glands moisturizes and protects hair. Sebum overproduction causes oily hair.
Straight hair becomes greasier overnight than curly or textured hair. Straight hair seems oily because scalp oils may readily migrate down it.
Your pillowcase may transmit oils and grime to your hair overnight, making it seem greasier in the morning. The friction and movement from sleeping with your hair down or untied may also disseminate oils.
How often and well you wash your hair might potentially cause nighttime greasiness. The scalp overproduces sebum to compensate for dryness when you wash your hair too often or use harsh shampoos.
Gels, mousses, and serums may leave hair oily overnight. These products weigh down hair and retain oil and grime, making it greasier over time.
Unhealthy scalps produce more sebum and greasy hair. Hormonal imbalances, nutrition, and scalp diseases like seborrheic dermatitis or dandruff may alter sebaceous gland activity.
Pollution and humidity may worsen oily hair. Pollution and humidity may make hair oily and frizzy.