Ask HN: daily emotional journal service?

I had an idea I think would be neat/fun to implement, but want to see if anything like it exists first. I was not able to find anything similar on google, but I would not be surprised if something similar existed and I am just searching for the wrong terms/keywords.

The idea is to implement a tool to help you track your daily mood. I was thinking of calling it "Good Day? Bad Day?".

You would get a daily email at a configurable time, and simply reply with "good", "bad", "neutral". You might be able to include an optional note explaining why you felt that way. You could log into your private profile on the website and see a calendar view of your past responses.

It would be very interesting to see historical trends in the data. If you felt "this was a really crappy month", then you could look back and see how often you felt that way and potentially identify patterns.

Do you know of any similar services or tools? Any suggestions about the idea? I think it would be a fun weekend project.