From Joost van den Broek on 2014/10/10 13:54:00 +0000
When starting sleep tracking, enable airplane mode (auto for root, pop-up non-root) on the phone and store track stats temporarily on the Pebble. After waking up and dismissing the alarm, disable airplane mode (auto for root, pop-up non-root) and reconnect to the Pebble to sync stats. Recorded sounds could also be synced this way. Maybe implement an option to do this automatically (if rooted) at the set smart alarm time, so smart waking up is still supported.
This combined with the new background tracking of Pebble FW 2.6 would be ideal 
Copied from original feature request: http://urbandroid.uservoice.com/forums/264867-sleep-as-android/suggestions/6546157-pebble-offline-tracking