# Sleep modes and waking up

A battery device spends most of its time asleep. What stays on, what wakes it up, and what is lost while it sleeps.

Page: https://dharanova.com/learn/low-power-modes · Track: Power · Level: Core · Facts last checked 6 October 2026

Read first: [Writing a power budget](https://dharanova.com/learn/power-budget.md)

Diagram: Bar chart, not to scale, of three sleep depths. Light sleep: some clocks off, high power, short wake-up. Deeper sleep: more clocks and regulators off, less power, longer wake-up. Deepest sleep: memory off too, least power, longest wake-up, and RAM contents are lost. Below, three triggers point at a sleeping microcontroller: an RTC alarm; a digital sensor that watches its own threshold and raises an interrupt line, a pin change, so the chip can stay asleep until something actually changes (a button can raise a pin change too); and a peripheral event such as an incoming byte.

## Key points

1. Microcontrollers offer several sleep depths; deeper modes switch off more clocks, regulators and memory, saving power at the cost of longer wake-up times and, in the deepest modes, lost RAM contents. [1][2]
2. Common wake sources are an RTC alarm, a pin change from a button or sensor interrupt, and a peripheral event such as an incoming byte. [1][2]
3. Many digital sensors can watch a threshold themselves and raise an interrupt line, letting the main processor stay asleep until something actually changes. [3]

## Where to go next

- [Designing for power that cuts out](https://dharanova.com/learn/unreliable-power.md): Power cuts and voltage dips are common in many places. A device should handle them safely and recover on its own.

## Sources

1. [ESP-IDF: Sleep Modes](https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-reference/system/sleep_modes.html), Espressif
2. [RP2350 datasheet](https://datasheets.raspberrypi.com/rp2350/rp2350-datasheet.pdf), Raspberry Pi
3. [LIS2DW12 datasheet (ST DocID029682 Rev 4)](https://www.farnell.com/datasheets/2581666.pdf), STMicroelectronics