Are you introverted, or do you need space away from a specific someone? I'm very much an introvert, so nearly every day I'll need time to myself, away from people, away from distractions, just to let my mind rest and recharge.

If you need time away from someone in particular, but are happy to socialize with others, it might be worth it to evaluate why you are feeling that way; should this person be in your life a little less or even at all? That's a tricky situation, especially if it is someone you care about deeply or have a close connection to. If you can figure out what prompted the desire to distance yourself, it gives you the opportunity to start a dialogue with them about it. If this is the case, and a conversation develops from it, using "I" and "we" statements is more effective than "you" statements.
Example using an outgrown friend: "I feel that our interests are more diverse than they used to be." is better than "You are interested in different things than I am."

If you are an introvert who just needs time to yourself, I agree with the previous replies; say that you are the type of person who needs alone time. It's not that you don't cherish the connection you have with them, but rather you require a disconnection from everyone now and then. It's in that alone time you are able to connect back with yourself. Anyone who cares for you should be able to accept that response.