You don't need an OS to do more advanced or efficient sleeping. You could have one of the timer peripherals generate an interrupt on a regular basis, and then put the chip in a lower power sleep mode in between interrupts.

Isn't that what an OS would do, essentially?

Yes, probably! But you can do it yourself without one.

Here is an example in C using libopencm3, that uses timers to trigger an interrupt used to flash LEDs https://github.com/libopencm3/libopencm3-examples/tree/maste...

that is basically a mini-os :)

You can set up a wakeup interrupt using a timer by moving a few values into a few registers. It doesn't require anything like an OS. Even tiny 8-bit microcontrollers can do this.

Sure, but putting the chip to sleep is one instruction, and programming the timer interval is only about a dozen instructions. So you don't really need an OS in this context.

There is a blurry line on the edges of "library" and "OS"; most people would put this particular example on the "library" side.