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★ Best answer
Leaving your phone on at night isn't a problem if it's not overheating and not draining completely. But frequent charging to 100% and discharging to zero actually damage the battery more than just keeping it on in standby mode for a long time. If you want to be as gentle as possible on your battery, keep it between 20 and 80% charge - that's way more effective than turning your phone off at night.
The battery degrades fastest from heat and full charge cycles, not from whether the phone is on or off. If it's sitting in standby mode at night at normal temperature, that barely affects wear - modern power management systems are well optimized. Now, if you leave it on with the screen lit or in a hot place, then yeah, that harms the battery way more.
# Translation from Russian to English
The main thing that kills a battery is heat and deep discharges, not just the fact that the phone is on! If it's just sitting quietly at night in standby mode and not heating up, degradation will be minimal. That said, if the phone is updating apps all night, syncing data, or running some process at full power - then the battery really does suffer from constant micro-cycles and heat.
There's one catch that people usually miss: the problem isn't standby mode itself, but the fact that a lot of people leave their phone at a high charge level (80-100%) overnight. For lithium batteries, storing them at full charge for long periods is stressful! If you're in the habit of plugging your phone in before bed and it stays plugged in all night charged to 100%, that really does wear the battery down faster than if it was just on with a charge of 50-60%.
The bottom line is simple: whether the phone is on or off doesn't really matter, as long as it's not heating up. What matters is temperature and charge level! If you want to be as gentle as possible on your battery, leave it on overnight but disable syncing and updates, or only charge it when the battery drops to around 20-30%.
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