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Welcome to EnviroDIY, a community for do-it-yourself environmental science and monitoring. EnviroDIY is part of WikiWatershed, an initiative of Stroud Water Research Center designed to help people advance knowledge and stewardship of fresh water. 
Hi Marion. If you haven’t received an answer for this yet, you could repost it in the Mayfly forum (http://envirodiy.org/forum/dataloggers/mayfly/). It will reach more people that way, as it will go out in the RSS email.
It really depends on what sensors you’re using. The board uses very little power while sleeping, so all of the power drain you should be concerned about is related to the sensors and how much they draw, how frequently you sample, and how many seconds you have to leave them on in order to take a measurement.