The 5+ Vitamin Concentrated Serum by Numbuzin is generally recommended for nighttime use. This is primarily due to its brightening ingredients, such as vitamin C, which can make your skin more sensitive to sunlight. Applying it at night allows your skin to absorb the product without the risk of sun exposure. Additionally, some users have mentioned that the serum can be slightly sticky or heavy, making it more suitable for nighttime use when you don't need to worry about makeup application.

Regarding compatibility with retinol, it's important to note that combining vitamin C with retinol can sometimes lead to skin irritation or decreased effectiveness of both products. If you're using retinol, it's generally advised to apply it in a different part of your skincare routine or on alternate nights to avoid potential conflicts. Always patch test new products and consult with a dermatologist if you have concerns about combining different active ingredients.