Include average go-to-sleep and wake up times to help make sense of (and manage) sleep irregularity info

From Chris Ball on 2017/10/22 07:45:43 +0000

Very difficult to make sense of present sleep irregularity chart and stats. I work shifts so will always struggle to improve irregularity but, when off work, could benefit from knowing where the mid-point is when going to sleep and waking up.

Copied from original feature request: http://urbandroid.uservoice.com/forums/264867-sleep-as-android/suggestions/31976002-include-average-go-to-sleep-and-wake-up-times-to-h