This was a weird thing to happen.
I’m not sure if it’s an iOS 17 thing, but the restart happened to both my iPhone and iPad.
My wife had exactly that happen now, twice. She’s on 17.0.3.
On the same iPhone 15, but still running the original 17.0.2 I have not yet seen anything like that. I know why I’m being extremely conservative with applying these updates.
If iPhones running 17.0.3 are actually off for several hours during the night, I wonder what happens if you have an alarm set or if that has any effect at all on this buggy behavior.
It’s happened to me every few weeks for at least several months. I saw it before and after installing IOS 17. However, it hasn’t happened in the short period since I installed IOS 17.0.3 last Wednesday. It happened on my iPhone 14 Pro and my new iPhone 15 Pro.
I find it’s one of the minor annoyances with Apple devices. I have Homepods in my living room and bedroom, and, most of the day, I use Apple Music on my desktop computer to play music or Internet radio through them. If I quit Apple Music and restart it, the Apple Music selection plays through the Homepods but not on the Desktop speakers. I need to uncheck and then recheck their selection in Airplay to get them going.
That’s interesting. Neither my wife nor I saw this before iOS 17. But that was also on iPhone 12s.
For me it was only last night. The only other times that I’ve seen an overnight restart was when my iPad ran an overnight iOS update, which I sometimes let happen, rather than running it manually when the update is ready.
I definitely saw the lack of bars overnight while both devices charged. Again, that I’ve never seen before this morning.
For some reason this sounds to me like the old daylight saving time bug that occasionally plagued iOS years ago.
iPad:
iPhone:
They both seemed to stop at different times.
Similar, but different. First time seen last night. Says temperature problem, and like the previous post, has a “Pause” symbol below the graph instead of a lightning bolt (charging?).
It didn’t happen with either device last night. I wonder if 17.0.3 was never intended to be released, but was instead internally testing this particular function on this particular date and they forgot to remove this test when they patched the overheating bug and the security issues?
I experienced the problem for what I think was the first time last night, though it’s possible there was one other time (I have a dim recollection of having to enter a passcode when I wouldn’t have expected to). This is my iPhone 15 Pro running iOS 17.0.3. No temperature issues on this one.
There’s two separate issues. Those folks who see the pause symbol under an unshaded area (as in @schwartz’s post) are seeing overheat protection kick in. Those suffering multiple-hour shutdowns of their iPhones are those seeing the shaded area with the flash symbol below (as in @ace’s post). These issues could well be connected through a common underlying bug, but their symptoms are rather different and mutually exclusive. A functional iPhone at your bedside (i.e. away from external high heat sources) doesn’t overheat so quickly that it emergency shuts off without first stopping to charge. And if it did, it certainly wouldn’t require 6 hrs just to cool off to ambient. If it did, that would be a sign it heated up well beyond temps that are compatible with flash pertaining its cells’ states, let alone the structural integrity of the device.
Edit: The pause symbol can of course also be caused by a pause in charging due to user having selected optimized charging, however, that pauses at exactly 80% and it does not lead to overheat warnings. When there is an overtemp warning like in @schwartz’s post, that is not optimized charging. That is the iPhone thinking it is getting too hot. Which for most people just charging an otherwise idle phone on their nightstand is of course extremely unlikely (even more so when it’s charging from >50%). The fact that it’s all of a sudden happening to huge numbers of users who never before had ay overnight overheating issues indicated it’s clearly bogus, i.e. a bug.
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My phone was not near an external heat source. It was sitting at my bedside charging, as every other night.
Exactly. It’s a bug.
We already know there’s a bug that shuts off others’ iPhones for no reason at all for multiple hours in the middle of the night. They are not suffering from overtemp. And you aren’t either. Your iPhone just thinks it is due to a bug. Chances are those two bugs are somehow related, but they clearly present quite different.
To get back to a question I initially had, some folks are reporting that their iPhones started up again (after multi-hr outages) just shortly before their alarm is scheduled to go off. 
Others are reporting they actually missed preset alarms. Yikes. 
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Are you using a MagSafe charger or is your phone plugged in with a cable?
My wife’s was attached to a cable both times it happened. She has in fact never charged hers over MagSafe.
MagSafe. As I’ve been doing nightly since MagSafe was available for iPhones. This occurrence was immediately or very soon after the 17.0.3 update, as others reported, and I don’t think I’ve seen it since.
Just as an update, this has not happened on my iPhone since that one time on October 10. It was the same for my iPad - until this morning, when I woke up to find that it happened again (this time the battery graph looked just like my iPhone graph in my prior post - just blank during the time while it was plugged in overnight, about an hour after plugging in; it also prompted me to enter my passphrase after restarting. That hadn’t happened with my iPad last time - it just looked like a normal prompt for the passcode after not having been entered for enough time.)
However, I was kind of surprised to find out that I hadn’t updated iPadOS in a while, as it’s still on 17.0.3. My iPad has automatic updates on, and it’s been 22 days since 17.1 was released - that seems wrong that it should go that long without a prompt to update. I’m about to manually update, but automatic updates do still seem to be weirdly not prompting me in a timely manner.
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Ditto for me.