Hi,
It all depends who that "somebody" is. If it's one of your parents or a friend you should be open enough and simply say: "Please be considerative, I would like to be alone now. I really need time with myself". If it's your boyfriend it's a differet story and you have to handle it with much more care (you must avoid him feeling it's an excuse to get rid of him). The best way is to say it simple and straight, something like: "I really love you and I feel very strong about our relations however there are times when I need to be with myself. Please don't read it differently, it's not a hint and has nothing behind it, I give you my word of honour, I really need 2B with myself". It should work.
Best regards,