`wait_for_next_tick();` in any microcontroller-based embedded system will put the CPU to sleep, often lowering your overall power consumption from the mA range it uses while actively running, to the uA range.

Some systems, like those based on coin cells, will completely shut down the chip, leaving only a single timer or input active, and when triggered will turn back on and initialise the entire CPU again to handle the input/timer. Some microcontrollers give you a small allocation of you can preserve under these conditions. That's how you get year+ runtimes on tiny batteries.

Edit: if you want to nerd out on this, I would start with Jack Ganssle's excellent guide: http://www.ganssle.com/reports/ultra-low-power-design.html