I found an interesting thread on XDA: what seems to be a bug leads under certain conditions to the Calendar app preventing the phone to go to sleep on HTC Desire. The known workaround is to remove Flickr sync.
Now that does not affect my phone but the method for finding out is interesting any may help you understand what is going on by analysing your battery stats. To do so:
*#*#4636#*#*Battery historyPartial wake usageNow if there is anything responsible for the partial wake in large proportions you should try to get rid of it as it is for sure counter productive: your phone's battery life is all about sleeping for long periods of time when you don't use it so whatever prevents that is not meant to be. These conditions are due by wakelock conditions, a status where an application (or the system) explicitely tells the system to stay awake.
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