Yes, neem oil can be left on overnight when used for various purposes, including head lice treatment or skincare. Leaving it on overnight allows the neem oil to penetrate and work effectively. However, it's essential to dilute neem oil with a carrier oil and conduct a patch test beforehand to ensure there is no adverse reaction or sensitivity to the oil. Also, consider covering your head with a shower cap or using a towel to protect your bedding or pillowcase from potential staining. In the morning, you can wash off the neem oil using a mild shampoo or cleanse the skin thoroughly.