The most likely cause of itching skin that occurs primarily at night would be some allergic reaction to something that you encounter when lying down. Do you notice any redness or rash? If so these make an allergy more likely. Common allergens that people encounter when lying down would include pet
dander and mites (which live in the mattress). If you have other allergy problems (like itching eyes or nasal congestion) you may also find that these are worse at this time for the same reason. Using a mattress cover may help, as may removing carpet from the bedroom and using a HEPA filter.
It may also be that you are having a reaction to something in the bed linens, such as a new soap or detergent that you are using to wash clothing. If you have recently changed cleaning products, you will want to think about this possibility.
It is also possible that the itching is simply from dry skin. For example, if you tend to take a hot shower before bed, this may be drying out your skin too much. Consider applying a good skin moisturizer right after showering.
If these approaches do not help, talk to your primary care doctor