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This was reported by someone else, not sure what exact version, but my assumption is the latest version of Mac OS X. Once they had NB 5.5 running, they indicated that sleep was not available. Not able to test myself as I am on a PC :)
I am sorry but, if you are notable to reproduce it and also you are not able to provide more informations, it's really difficult to solve it ;( We need as much informations as you can provide ( please look at http://qa.netbeans.org/bugzilla/reportbug.html ). What JDK version, .... ? BTW: what do you mean by "going to sleep" ? Do you mean minimize ?
without additional informations for long time - closing.