Cutting the photoperiod isn't as effective as shading the light with slow progression to full light exposure. If you shorten the photoperiod your merely decreasing the length of time your exposing your livestock to intense light that they are not used to (a sun burn for a short length of time). If you shade with several layers of screen, frag rack, etc.. and raise the light, you can slowly take away the layers and lower the light allowing the animals to slowly adapt to the intensity. The animals didn't originally have trouble with the length of the photoperiod so when you change the photoperiod and increase the intensity your really causing the animals to stressfully photoadapt to two different changes. They will need to adapt to the intensity and alter their metabolism to accomodate a shorter photoperiod. This will likely cause them to brown out as the zoox are increased in an attempt to take in light while they can. At the same time the zoox are being burned the short time that the light is on since they aren't adapted to the intensity (this leads to less effective photosynthesis and metabolism ultimately affecting the host coral which has a mutualistic relationship with the zoox).
Jeremy