Automated sleep log

I would like to create a device that will log when a person falls asleep. Of course, someone can't just open a software开发者_运维技巧 application and make an entry say "fall asleep, 10:13pm" and be asleep a few seconds later. Instead, I was thinking about hacking a blackberry to log whenever a person powers it on to check the current time. The specific algorithm is not important, but is it possible to write a piece of code be written to intercept the power on button and write the current time/date to a file? If so, how is it done?

Also, if anyone has a simpler idea, please share.

I haven't tested it, but since you're asking for ideas:

You have your application running in background (or even an app which doesn't extend UIapplication) and have a Task (using Timer and TimerTask) that repeatedly checks if Backlight.isEnabled() returns true. If it does - somebody is using the phone. You can even incorporate an AlertListener class to check when the user has been woken up ;)
The downside of this solution (if it works) is that it is something of a 'busy waiting loop', so intercepting some event would be much better.

As far as writing down the current time is concerned - it's possible and sample code snippets are everywhere, you can of course use the persistent store or an SQLite table to aggregate the results in an interesting way.

Funny thing is I've been thinking about an app like this lately - it might be an good idea.