The Prune Danish via Amazon offers Prime members the Gear4 Renew SleepClock Sleep Monitor for iPhone / iPad / iPod Touch, model no. PG537, for $12.79 with free shipping. (Non-members can spend $35 or more to bag free shipping; otherwise, it adds $8.02.) That's $137 off and the lowest total price we could find. It measures how long you spend in deep, light or awake sleep phases, your breathing rate and movement, and your sleep efficiency.