Mac Mini powers down instead of sleeping. Performs full restart when awakened via keyboard.

On discovering that my Mac Mini is using more electricity than our refrigerator, I decided that I really should try to use the sleep feature. I find that my whole setup uses about 80 watts when the Mac is awake, 25 when it's asleep.

Unfortunately, when I set Energy Saver to the defaults--turn off display after 10 minutes, put hard disks to sleep when possible, wake for network access, enable power nap--and, in the Screen Saver panel, uncheck "Show screen after..."--the Mac will not stay asleep, ready to be awakened. After some period of time, it seems to power down. Hitting keyboard keys, though, seems to power it up--at any rate, it goes through a full "restart" sequence, taking several minutes. Furthermore, during the restart, several apps complain that they were shut down improperly (BBEdit and Telegram being two of them).

Why is the Mac powering down during sleep, and how can I prevent this?

macOS Monterrey 12.6.3

Mac Mini late 2014 (Intel)