Have a look at it and see if it has ICP printed on it anywhere. You say it is usually this one, do others trip as well? When this one trips I'm guessing you lose all your power correct? How old is the wiring?

There should be 3 different breakers;
The first, often on it's own and possibly with a seal so you can't change it is the ICP (Interruptor Control de Potencia).  This is a legal requirement as it controls the amount of power you can take and should be set to the power that you have contracted with the power company. They recently publicised the fact quite widely in newspapers, radio and when they sent out bills so it sounds like they are going to tighten up on it. If you don't have one and they come to check, expect a fine!

The second is the IGA (it doesn't say that on it). It was introduced a few years ago so is only in newer installations and isn't mandatory.

The third is the differential (earth leakage circuit breaker). This has a test button on it and is designed to trip if there is a leakage to earth such as someone putting a finger in a socket. Almost all houses have them but I've seen a number that don't work - if you press the button and it doesn't trip it needs replacing.

If any of these 3 trip you should lose all your power - if not then something has been wired wrongly.

After these 3 are the individual breakers for the individual circuits.

So, in your case, what is tripping? If it's the ICP then something somewhere is taking too much power - hot water heater/pump/cooker/something else? If it's only ever at night then that's odd. There are power fluctuations but not that should cause you to take too much power. How regularly does it happen? Do you know what time of the night or is it random? Do you have something set on a timer to come on at some early time?

What is the rating of your ICP (assuming you have one)?