the healthiest alarm-clock I know-of is simply to have 2 lamps on wall-plug timers, such that the warm-white one comes on 1st, then say 5mins later, the daylight one comes on.

Given how the beginning of sunshine changes through the year, you have to change these timers every couple of weeks, or 1/2-month, but that's a small price to pay for quiet, biochemistry-altering waking up that actually works properly.

Not an app, but it is an alarm-clock, and it wakes one up well, using the very skin-light-sensing system we are evolved to be using, instead of using some noisemaker, as "normal" alarm-clocks do.

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