Sorry. I started skimming too soon. 
Several of my computers run SyncToy early each morning. I use Windows Task Scheduler to wake the computers about five minutes before Task Scheduler runs SyncToy. Works fine. The computers never leave the log-on (PIN) screen state before going back to sleep afterwards.
That sounds like Hibernation (https://www.howtogeek.com/102897/whats-the-difference-between-sleep-and-hibernate-in-windows/):
Hibernate mode is very similar to sleep, but instead of saving your open documents and running applications to your RAM, it saves them to your drive. This allows your computer to turn off entirely, which means once your computer is in hibernate mode, it uses zero power. Once the computer is powered back on, it will resume everything where you left off. It just takes a bit longer to resume than sleep mode does. Sleep mode used to be much faster than hibernation, since hibernation was limited by the speed of a mechanical hard disk. Modern SSDs — especially PCIe NVMe SSDs — are blazing fast, and it really shows when you’re using hibernate.
Sleep mode is a power-saving state that is similar to pausing a DVD movie. All actions on the computer are stopped, any open documents and applications are put in memory while the computer goes into a low-power state. The computer technically stays on, but only uses a bit of power. You can quickly resume normal, full-power operation within a few seconds. Sleep mode is basically the same thing as “Standby” mode.
I suspect that the OP has his System Power mode set to Hibernate instead of Sleep, and the Countdown Timer won’t wake the box from Hibernate.
Here in the UK, British Telecom provide a handy phone alarm.
I don’t know why but the SDMB won’t always allow an *.
The first action below has an * before and after the 55.
550730# means phone me at 0730
#55# means switch off any current alarm
*#55# means check if any alarm has been set
With the caveat that I’m talking about Windows 7 – though I’m pretty sure that later versions are the same – this is absolutely false. When I select “sleep”, I can actually see the system doing a whole bunch of disk activity before it shuts down. More definitively, there has occasionally been a power failure when one of my computers was in sleep mode. When I restart, instead of coming up immediately as it normally would, it goes through the normal boot process, but then displays “Resuming Windows” and the computer is exactly as it was before, retrieved from the saved context on disk.
ETA: @glee 2 posts up.
A * somewhere mid-paragraph means start or finish italics. Double-* means start or finish bold. If you have just one * in a paragraph, the formatter can correctly figure out you mean to display an asterisk. But if you have 2 or more, something is going to be italicized instead.
Further, starting a paragraph with a * means that you want a bulleted indented list.
If you want to display a plain * anywhere in any paragraph, precede it with a backslash like this example: \*. That always works. That’s how I displayed all the asterisks in this post.
That suggests to me the machine you’re running on is old enough that it doesn’t support “sleep” and uses hibernate as a substitute. Lots of desktops didn’t do sleep back in the day; that was a laptop feature.
Windows sleep & hibernate have evolved quite a bit over the years (and I don’t know the full details anymore). For example, there is such a thing as “hybrid sleep”, where it’ll both write RAM to disk and enter a normal sleep state. If you wake it normally, it’ll just resume from RAM. If the power goes off, it’ll resume as though it hibernated. There may be other modes I’m not familiar with. ACPI power states have also gotten quite a bit more complicated than the 90s… (I also don’t know them)
So you may both be right!
Edit: And I think it will get even more complicated in ARM chips in the latest Macs and Snapdragon PCs — as usage patterns change and battery life becomes more important and competitive, the lower-power states have gotten more complicated and generally “better”, but also harder for the end-user to predict and understand.
I regularly sleep my Win11 laptop whenever I get up from my desk. I’ve never investigated any programs that could wake it deliberately, but my laptop absolutely can and will wake itself up, unwanted and unbidden. The usual reason, as best as I’ve been able to tell, is because I’ve got uninstalled updates waiting. (I don’t have it set to let it install updates automatically, because I frequently leave my laptop running an overnight job that I don’t want interrupted with a spontaneous reboot.) Whatever the reason was for the unbidden wakeup, it never tells me why. It isn’t always because I’ve got updates waiting.
Yeah, “why is my machine an insomniac” is one of the harder problems to diagnose! Everything from a bad USB driver to the cat moving the mouse to a stray network packet to queued updates etc. can wake the thing and the OS never deigns to tell you why.
You can use the secret Windows SleepStudy tool to better understand when and why the computer sleeps and wakes: Modern standby SleepStudy | Microsoft Learn
Thanks - that’s helpful. 
Trying to get your computer to sound alarms through sleep will have all kinds of weirdness and gotchas. If you’re tech savvy then the problems aren’t too bad, but it sounds like you’re not at that level. For your use case, it would probably be best to use your phone’s alarm. That will be simpler and more reliable than depending on your computer. Plus, the phone is with you when you’re not at the computer. If you’re in the yard, garage, at the store, etc., you’ll hear the alarm on your phone. Whatever phone you have will almost certainly have an alarm feature. We can look up how to do it if you tell us the model of phone you have. Also, look for the timer function. Often, a timer will be more what you need. If you need to remember to take the pie out in 50 minutes, it’s easier to set a 50 minute timer rather than figure out an alarm that’s 50 minutes in the future. The alarm feature is more useful for when you need to wake up early to prep the house for the pie party for all your SDMB friends 