Personally I don't have the useful military pre-requisite of physical exhaustion so I just use sleep-phones and Audible with a 30 minute sleep timer and somethi

Personally I don't have the useful military pre-requisite of physical exhaustion so I just use sleep-phones and Audible with a 30 minute sleep timer and something engaging but not too exciting to listen to; eg. a good history of something undramatic. Slow the reading pace down to match your breathing cycle, and its really effective.