It's dangerous to leave the 3DS in sleep mode for a long time? I almost never turn off my 3DS
I don't recommend using sleep mode unless you're in a middle of a game and can't get to a save point. When not in use, turn off the 3DS/2DS and don't leave it plugged in with the AC charger once it's done recharging the battery. For long term storage, charge the battery at 50%, remove it from the system, and store it in a dry/cool/dark storage area. Although the risk is extremely small, a battery left in the 3DS can trickle charge wearing out the micro-fuses, and if it's near end of life expectancy/has bad cells, it can swell or explode damaging the case and cosmetics. ← Might as well contain the damage to the battery alone than come back finding your baby is fugly.
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While there's no proof prolong use of sleep mode will damage the 3DS system, several long time members here had the displeasure of troubleshooting the once deadly Black Screens of Death with working sound in HOME Menu as was referenced by
@lone_wolf323's post #7.
What was so baffling about this bug was that some users reported putting their 3DS in long term sleep and then flipping open the screens to find theirs in a comatose state. It was later determined (thanks to
@gisel213 discovery) that a failed camera module out of all things caused this bug that seemed like a software issue with the 3DS firmware. This is why troubleshooting 3DS/2DS systems requires taking into consideration both hardware and software aspects because of their poor redundancy when components fail.
@Extrasklep, you now know why I gave you a hard time about your o2DS touchscreen and cameras in your thread.
This is just my theory, but I think the cameras are still functioning while the 3DS is in sleep mode. If this is true, sleep mode causes an expedient rate of wear on either front camera failing to focus on seeing up close, front IR LED burning out (n3DS and n3DSXL models only), or whatever with the two (2) rear cameras saw that freaks them out (spooky ghosts?).
Does sleep mode ~ camera wear (if it exists) apply for every 3DS or 2DS systems? Eh, beats me.