Sorry for the pedantry, but the moon is up during the day as much time as it is at night. Sounds like a bug in the timing routine. That or they're taking advantage of tidal effect on hydroelectric systems....

There are different types of power stations, those that are considered base load and those that are considered peak. The base load generators are stuff like nuclear or coal plants which are very inefficient to start up and shut down and thus are kept running 24x7.

The peak load power stations, which are typically natural gas or oil fired, cover the difference between the lowest demand points and the highest by varying their power production.

So if you move your demand to the baseload hours, you:

1) avoid running during the hottest part of the day where the cooling demand for the datacenter is the highest

2) move your load from natural gas or oil fired power stations to nuclear or coal (depending on the CO2 output of the specific coal, natural gas or oil fired generators in question, this may or may not help, but it would definitely help if they covered the base load by adding nuclear plants)

3) by avoiding additional peak load, you avoid or delay the construction of new peak load power plants, and the construction process with all the equipment, people, materials, etc. is probably responsible for a lot of CO2 in its own right

Something like 75%+ of the worlds oxygen comes from the ocean, how does planting trees really help?
Ever though cheaper electricity at night is a good idea, you're still emiting carbon.

Trees do not offset CO2, for whatever amount of CO2 they convert to O2 during the day, is released overnight. Moreover, when they die, they release the CO2 that it absorbed from the soil.

Perhaps environmentalists should go back to their elementary school books and review the CO2 cycle.

at night much of the energy produced by power plants isn't used? So by shifting servers to running only at night time, you mop up wasted energy and lower the overall requirement for energy during the day time.

Could be wrong.

...or should we say collective hysteria... It's really depressing to see how "carbon" seems to have become a bad word. Are people forgetting that life on Earth is cabon-based?

A little teaser:
http://scienceandpublicpolicy.org/monckton/goreerrors.html

And a free bonus:

http://scienceandpublicpolicy.org/sppi_originals/fallacies_about_global_warming.html


Perhaps they mean when it's darker it's colder, so cooling costs are lower?

You had better learn to read a little biology.

Trees do indeed sequester CO2, and measurements (from experiments, familiar with the concept?) show that they don't release nearly all the CO2 they gathered in the day. If they did, they wouldn't get bigger...

To further the explanation, had you ever encountered a dead tree, you would discover you would run into it and hurt yourself, as they don't instantly disintegrate into gaseous components.

"though it is unclear how this saves power."

Well, everyone knows money = power, right? I mean, it's clear the same server would consume exactly as much power (not power brokers though) at any time of day. Though maybe less heating/cooling for the pawns of the powered running the installations?

One of the big costs for a data center is cooling. At night it's most often cooler, and there is no sun load to fight so cooling costs should be quite a bit lower. That would cut power use, not just cut costs. On the down side you would need more data centers.