For me I would forget to do that every night. It would need some reminder to open the lens before bedtime.
Since no one has mentioned it yet, SleepAnalyzer (http://wiki.maemo.org/SleepAnalyser) has a sleep cycle alarm (and music I believe - for lucid dreaming) that wakes you up with a "slow" alarm. No lighting integration, but personally I doubt it would be pleasant to wake up to the N900's LED lights anyways.
I used to, but that has changed since I started using the N900 as an alarm clock. The internet alarm clock script I wrote has been the most reliable way of getting me to wake up that I have ever had. I am afraid now of separation from my N900! The secret really was to just have classical music come slowly up & play for 15 minutes before the regular alarm goes off. Apparently the fade-in slowly wakes you up enough to not need to abruptly just hit snooze when the alarm goes off. I would have put it together as an app, but was never able to set up a successful programming environment.