Some people might have an impression of robots as tireless workers capable of doing a repetitive job endlessly and accurately. But robots have limitations just like anything else. The Mars Phoenix Lander is one such example, for it only has a limited amount of power to work with, and if things go wrong, there isn't much it can do to fix itself, although the ground operators for the lander back on Earth would still try their hardest to work around any problems. It is also not infintely smart; the lander is actually fairly dumb with only a limited amount of memory.
So I guess the limited amount of power provided by its solar panels and the limited amount of memory it can use to store data before having to send it to Earth is the reason why the Mars Phoenix Lander hasn't worked on 24-hour shifts until now (except it's more like a 25-hour shift, since a Martian day is closer to 25 hours long). The lander has for the first time worked all through day and night, using a fork-like probe stuck into the Martian soil to continuously measure how well heat and electricity is conducted, using only power stored in its batteries during the night. This could help determine how the water in the soil changes between solid and vapor as the temperature changes throughout the Martian day and night. They don't expect the detection of liquid water since the region is too cold for it, though.
Soon enough, Phoenix will also be initiating a direct chemical analysis of icy soil it scoops up from the Martian soil. Hopefully, we won't have problems like the last time it tried to analyze the soil, where the soil was too clumpy to pass through the screen protecting the lab instruments. And if we're lucky, it won't be the last direct chemical analysis it does if it can avoid a serious short circuit in the instruments.