Overworld Fatigue

# Pokemon become fatigued over time, and this reduces their natural ability to heal over time while you walk. Overly fatigued pokemon have a chance of falling asleep outright. Fatigue is only cured with a heal at the Pokecenter, or a Sacred Ash. Really anywhere the heal method is specifically called. Credit to Riviera for the idea, and I'll probably edit this with the proper styled header once I have their approval for the implementation.
FATIGUESTHEALFRACTION     = 16 # 1/16 of total hp healed as a baseline.
MAXFATIGUE                 = 100 # the maximum value of the fatigue, which also is used to make a percentage multiplier for the hp healed
FATIGUETHRESHOLD           = 30 # 30 fatigue to have chance of sleeping
FATIGUESLEEPCHANCE          = 4  # 1/4 chance of falling asleep

class PokeBattle_Pokemon
  attr_accessor(:fatigue)

  def fatigue
    @fatigue=0 if !@fatigue
    return @fatigue
  end

  alias __fatigue_heal heal
  def heal
    self.__fatigue_heal
    @fatigue=0
  end
end

Events.onStepTaken+=proc {|sender,e|
   next if !$Trainer
   for pkmn in $Trainer.party
     next if pkmn.egg?
     pkmn.fatigue+=1
     pkmn.fatigue=MAXFATIGUE if pkmn.fatigue>MAXFATIGUE
     if pkmn.hp != pkmn.totalhp
       pkmn.hp+=(pkmn.totalhp/FATIGUESTHEALFRACTION*(1-(pkmn.fatigue/MAXFATIGUE))).ceil
       pkmn.hp=pkmn.totalhp if pkmn.hp>pkmn.totalhp
     end
     if pkmn.status==0 && pkmn.fatigue>FATIGUETHRESHOLD &&
         rand(FATIGUESLEEPCHANCE)==0
       if !(pkmn.hasAbility?(:VITALSPIRIT) || pkmn.hasAbility?(:INSOMNIA) ||
            pkmn.hasAbility?(:SWEETVEIL) || (pkmn.hasAbility?(:FLOWERVEIL) && pkmn.hasType?(:GRASS)) ||
            (pkmn.hasAbility?(:LEAFGUARD)&& $game_screen.weather_type==PBFieldWeather::Sun))
          pkmn.status=PBStatuses::SLEEP
          pkmn.statusCount=3
       end
     end
   end
}