Cvstable: No the fluctuation won't hurt the eggs. Goal is 45% through day 18 then up to 65%. My humidity fluctuated between 15 - 50% in the first stage, and I had an excellent hatch. Keep an eye on your air cells, and compare them to the charts. That'll help you stay on target. And yes, you will see temp fluctuations even though your bator is thermostatically controlled. When the chick's metabolism increases, you might see some temp spikes and may need to adjust temp accordingly. Roadeo99: I'd be more interested in the humidity inside the incubator. I understand the concept of keeping the humidity up in the room the bator is in, but honestly, it's what's going on in the bator that really matters. Remember that chicks need air. It's tempting to block the air vents to the bator to keep the humidity up, but that also cuts down on O2 availability.