Itchy during summer nights, especially when I sleep without shirt. However the rest of day I do not itch at work. I see google said itching can be sign of liver disease, I take xanax (alprazolam). But feel 100%?

It is possible. That the itching represents a skin sensation related to sedative use. This is especially true if its been more than 6 hrs from last xanax (alprazolam) dose. I am not saying this is the reason, but may contribute to intensity of symptoms as well.
Heat rash, allergy? This could be caused by heat rash, allergy to things in the environment, etc. doubtful this is due to liver disease. Your doctor could help you figure out the cause. Good luck!
Get . Full blood work to check on things. From what you say I doubt liver disease. May be allergies. Could be meds. Talk to your doctor. May help find answer.