I know that papayas prefer it pretty hot (& humid), it's possible it doesn't like the nighttime temperature drop. From what I read when trying to germinate some papaya seeds, they can be very finicky about cultural conditions in general.

It's also possible that it's a drought response, which could happen if it's staying too wet, although I would assume with daytime temperatures that hot it would still dry out quickly enough for that to be less of a problem.

In general though, leaves turning yellow implies that there could be a nutrient deficiency. Nutrient deficiency could be caused by lack of nutrients in the right quantities in the soil (usually zinc/iron/copper for chlorotic leaves depending on which part turns yellow first), or the nutrients being unavailable for uptake, for example if the soil has become too acid or alkaline (if I recall, papayas like it more acidic, but don't quote me on that).

It does sound like the leaves are turning color and dropping off fairly quickly, which I've seen happen to some plants that were definitely nutrient deficient.

What kind of pot/medium is it in? How long has it been in the same pot/medium? What's your watering/fertilizing regimen?

I'm kind of a novice and have never successfully grown a papaya tree, but hopefully I'm asking the right questions for others to chime in, if nothing else!

I recently had the same problem with a Papaya raised from seed. It was doing well and all of a sudden the leaves began turning yellow and falling off. New leaves were tiny and it only had 3 or 4 left. I was ready to remove it assuming that it would die but at the last minute decided to fertelize and water heavely and see what happens. It came back almost immediately and has set new dark green leaves and is healthy again. I live in Central Florida and this is all new to me having come here from the Catskill mountains of N.Y. I'll continue to fertelize regularly and water every two days or so and hope for the best. This approach may be worth a try for you also.
good luck,
tom

Don't know how often you fertilize, or with what; but they require very heavy feeding of nitrogen rich fertilizer. I have been using straight, fresh, chicken litter monthly. I usually add a couple pounds for each potted plant. Outside, planted in the ground, I put about 2" deep all the way around. They grow faster than bamboo (nearly) and never burn.