Battery is the silent killer of always-on mobile sensing. If your app records or samples through the whole night, you are one bad release away from a 40% overnight drain and a one-star review that just says "killed my battery." Some hard-won tactics.
The budget mindset
Treat energy like a memory budget. Every wakeup, every sensor poll, every network call has a cost. The goal is not zero cost - it is predictable, disclosed cost. Users forgive "uses ~8% overnight" if you tell them; they do not forgive a surprise.
Concrete tactics
- Batch, do not stream. Buffer samples and process in chunks instead of waking the CPU continuously. Coalesced work is far cheaper than the same work sprinkled across the night.
- Downsample aggressively. Full-resolution capture is rarely needed end to end. Capture high, immediately reduce to the features you actually use, discard the rest.
- On-device beats network. Classify locally and upload summaries, not raw data. Radios are among the most expensive things you can touch; a nightly sync of small summaries costs a fraction of streaming.
- Respect thermal and low-power state. Back off when the OS signals low-power mode. Fighting it drains battery and gets you throttled anyway.
- Measure on real devices. Simulators lie about energy. Profile an actual older phone across a full night before shipping.
Why it is worth the effort
Overnight sensing only delivers value if users leave it on, and they only leave it on if it does not cost them their morning charge. For sleep specifically, the payoff is capturing events other sensors miss - the acoustic ones like snoring and breathing pauses covered in this overview of snoring vs. sleep apnea.
What is your worst overnight-battery war story?
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