Just wanted to share this with other users, if you would like to get your whisper node into sleep mode for a specific time you can use the lowpower library from rocketscream:

Repo: http://github.com/rocketscream/Low-Power

More info: http://www.rocketscream.com/blog/2011/0 ... o-library/

It allows you to set the mcu to sleep for maximum of 8 seconds, if you want to sleep for longer times you can keep a counter so it will immediately go to sleep again untill the counter reaches it's value to start doing something.