Are the lights on a timer? If not you should really consider getting one. All fish are startled by the light, especially if a large person...to them, is turning the lights on, but if on a timer they get use to the "norm", such as when the sun comes up in the morning at certain times your body is use to the light shining in the windows. Also a light timer is always at work, even if you decide to sleep in.
If the fish is new to the tank, this is a usual occurrence. Give the fish more time to adjust.