Well, you could create a file called log.txt (or whatever you want to call it), and after every read from the sensors you have to add a system call from Node that tells the the read value to be stored on log.txt. In oreder to make a system call from Node you have to use Child Process. I'd suggest you to take a look at Child Process Node.js v0.12.4 Manual & Documentation
Other options for doing this are to use the Arduino IDE or the Linux side. But from Node this is the only way I can think of.