I recently brought my Bougainvillea indoors the leaves are falling off. I water when it dries and it has sunlight.
Leaves Falling Off Bougainvillea
Certified GKH Gardening Expert
Bougainvillea like their soil to be moist, not soggy. Do not allow the soil to dry out.
No matter how much sunlight indoor the plant is receiving it is less than the outdoor natural sunlight.
The plant is likely stressed from the change in light.
Water the plant enough to keep the soil moist, but make sure the pot is well draining.
Here is a link to refresh you on the care requirements.