I don't know if this is a problem or not. Some back ground on the set-up first... temperature and humidity are measured by Acu-rite digitals; one on the warm side and one on the cool side. The cool side is left unheated so I have a temperature gradient. The warm side is heated with an 8-watt undertank heater and a 50-watt red IR bulb. If the IR bulb doesn't get the temperature in the high 80s / low 90s, I switch it to a 60-watt Daytime bulb. Night times get a blue "moonlight" 75-watt bulb.

Here's the problem... my ball python leaves her hide whenever the temperature gets above the high 80s. Seems like anything above 85 is too hot. The thermometer is right on the substrate (aspen) so I don't think she's getting burned. She'll go over to the cool side (where it's around 75 degrees) and hang out until I unplug the undertank heater or change to a lower wattage bulb.

Can a small (24" long) ball python be healthy in mid-80 degree temperature as the high? I'm a bit confused as to why she doesn't like the heat.

EDITED to add: she's showing no signs of illness, and is eating just fine. Shedding was about three weeks ago, and humidity is around 50% since I've added sphagnum moss. She's active at night as usual, but still likes the cooler side of the aquarium.