The NuttX power management has four levels or modes (from include/nuttx/power/pm.h):
NORMAL
The normal, full power operating mode.
IDLE
This is still basically normal operational mode, the system is,
however, IDLE and some simple steps to reduce power consumption
provided that they do not interfere with normal Operation.
Simply dimming a backlight might be an example something that
would be done when the system is idle.
STANDBY
Standby is a lower power consumption mode that may involve more
extensive power management steps such has disabling clocking or
setting the processor into reduced power consumption modes. In
this state, the system should still be able to resume normal
activity almost immediately.
SLEEP
The lowest power consumption mode. The most drastic power
reduction measures possible should be taken in this state. It
may require some time to get back to normal operation from
SLEEP (some MCUs may even require going through reset).
Alan C. Assis 