Laptop-mode

An often overlooked feature in the modern 2.6.6+ series of Linux kernels is the laptop-mode. It may be activated by writing a "1" into /proc/sys/vm/laptop_mode. Laptop-mode, when configured correctly, can make the kernel buffer disk activities for quite a long time and keep the harddisk spun down for most of the time to save power.

There is also a set of userland tools made to automatically manage all aspects of laptop-mode configuration according to the actual mode of operation (ac/battery/status). It is called laptop-mode-tools and you can install it in debian via apt-get or download it from here.

Almost anything you need to know about laptop-mode can be read in your Linux kernel documentation at /usr/src/linux/Documentation/laptop-mode.txt
If you can not find this file on your laptop, then go and get yourself the latest kernel sources.