Would Heart of the fields allow a character not to be fatigued from sleeping in armor? Even Heavy armor per se?
Yes, it prevents fatigue or exhaustion once per day regardless of the source.
Heart of the Fields is a superb effect; any fatigue or exhaustion can be cancelled. Resting in armor, magical spells, environment effects, barbarian rage; it covers them all. The catch is that it's only once per day. Which one effect are you ignoring fatigue/exhaustion from? Some of these things would incur checks more often than once per day; you can only ignore fatigue/exhaustion once. So if you're in an adverse weather condition that checks for fatigue once per hour, Heart of the Fields only saves you from your first failed check.