What triggers Smart Wake to go off on Sense 2?

[SunsetRunner]

I try to use the smart wake feature for the alarm to wake me up some mornings, but it rarely goes off in the 30 minutes prior to the actual time I set it for. Sometimes, I'll even be lying awake in bed and it still doesn't go off (I double check it's set to smart wake). What triggers it and why does it not seem to work for me? 

Moderator Edit: Clarified subject

[sgbednar]

Mine seems to be triggered by movement.  If I wake up, it won't necessarily go off right away, but if I roll over it will.  

[Rich_Laue · Accepted solution]

This feature is suppose to be looking at sleep stages, and avoid going off durring a deep sleep period. 

See How do I manage Alarms