Why is My Green Spotted Puffer Fish So Odd?

I have 2 green spotted puffer fish in a brackish tank with a gravity of 1.007. They are both about 1.5" in a 25g, with a Marineland Bio-Wheel 150 filter. During the day when the light is on, they are active and happy. But when I wake up in the Morning after their light has been off for 8 or so hours they are both in the corner inside the cave in the corner, motionless and very pale. Then when I turn on the light, within 30 seconds they spring to life swimming normally with bright color again. It happens so fast that you can actually see the spots returning to their body in seconds.

Does anyone esle see their GSP do this? is this normal?
Please dont put your answer only that my tank is too small, they are very small to and i will upgrade when they get bigger.
 
They're "sleeping". Most diurnal fish will do this, when it's dark they'll go into hiding and will lose a lot of colour. My little endlers do the same, they snuggle up in the plants and on the sand and lose a lot of colour and look at me VERY grumpily if I turn on the light!
 
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