There's a new feature in Gmail Labs called Sender Time Zone that can help. Turn it on from the Labs tab under Settings, and you'll see green phone icons next to people who are probably awake and readily reachable (if it's between 9:00 am and 6:00 pm in the sender's local time zone) and red ones next to those who could be sleeping or out of the office:

Source: gmailblog.blogspot.com/2009/04 ...

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Pretty soon we won't need to think anymore. (This could be a good thing, depending on the soundness of your current thinking.)