it's dust particles. about 99% of the time obs in pictures is dust. most people who believe in spirits don't find pictures of orbs as credible evidence. why you see more orbs at night instead of day is the amount of light. if your flash is on it'll pick up on the particles, same if the sun is setting, the light is angled in a way it reflects off the dust in the air.
if you take photo's of the same spot all in different settings, and orbs appear in the same spot [roughly] each time, and your lens is as clean as you say, then you may have something. as it stands, orbs aren't much of anything. you could try with a video camera, or meditate where the orbs tend to appear, see if you hear/sense anything.