I had a professor once tell me that it doesn't matter. The advantage of an email is that they can read it whenever they want. I once sent her an email like at 2am and apologized for the time, and she said never to apologize for the time in an email because it's not like you're calling the person or knocking at their door. Just my 2 cents. 🙂 I've sent thank-you notes on weekends, at night, you name it...😀