[olliric]

It's real fun and very different from daytime. Most fish are in a sleep state and hidden somewhere. There is a lot less activity and very few fish are swimming about. Some species like lobster, crab, heel and octopus are more active at night and venture out of their daytime retreats and can be easily picked. Occasionally you can find some fish that is sitting in the open as if it did not find shelter before the night fell. It's still unclear to me if the fish just sits there for the whole night or actually moves about, feeds and only stops when the powerful light beam hit it. After having caught some of the same sleeping fish on the hook I think that they do feed and swim about but at a much slower pace.
If you want to spear fish you should pick a night with good viz, stay shallow and within the maximum range of your dive light (I use a Technisub Vega 2 with a 10w bulb and fish 3 to 10 m deep). You need a short and not very powerful gun. You always shoot the fish execution style. The fish usually can be approached at very close range. In some cases if you have somebody holding the light you can actually grab them with your hands.
Swim on the surface and swing the light around to spot some target. Even better is to dive and swim just above the sea floor and sweep the light beam in every hole looking for some sleeping fish. You can find holes with lots of them just sitting still.
Ideally you want to dive with another person that can assist you taking the fish off the spear, load the gun ect. (because of the light if you are by yourself it's like having only one hand). It is also handy to tug a little boat/inflatable to drop the catch on, especially if you are hunting in shark territory. Be aware that because of the light beam refraction it is difficult to aim accurately at the target. I have missed fish just inches away from the spear tip!
Beware, in some countries especially in the med is forbidden to spearfish at night.
The atmosphere at night is unique and very intriguing. Because of the darkness everything appears muffled and calmer than in the day. Apart from the odd darting bait fish the marine life sleeps.
Because of the very narrow visibility of the light beam, you may have a 6 m shark 1/2 m away and not see it. That adds to the "mystique" of night diving and spook some people off.
Despite having dove at night in the tropics I never encountered sharks. In general you can get plenty of fish rather easily and I did catch a record size spiny lobster and crab
Cheers
Giovanni