Why did my computer go to sleep anyway?
Almost always the lid was closed, the time ran out, the battery reached 20%, the app was quit, or a scheduled shutdown ran, each with a sign you can check.
Keep Awake stops your computer falling asleep on its own. A few things still end it, on purpose, and each one leaves a sign you can check.
#The lid was closed
Closing a laptop’s lid puts it to sleep, and Keep Awake does not change that. Leave the lid open while the task runs. A Mac can also keep running with the lid shut when it is plugged in and has an external display, keyboard and mouse, as Apple describes; on Windows the lid action is a power setting you can change.
#The time ran out
Five minutes before the end, a notification said Keep awake ends in 5 minutes, and at the end another said Keep awake has ended. Next time, choose a longer length, or Until I turn it off, or press Add another hour in the menu bar when the warning arrives.
#The battery reached 20%
On battery, Dictate Safe warns at 25% with Battery is getting low, and at 20% lets go with Keep awake has stopped. That is deliberate: a laptop that runs flat shuts down uncleanly and can take unsaved work with it. Plug it in for long tasks.
#Dictate Safe was quit
Keep Awake belongs to the running app. Quitting it, or restarting the computer, ends it, and it does not switch itself back on the next time Dictate Safe opens.
#A scheduled shutdown or restart
A shutdown or restart that your computer, or your organisation, has scheduled still happens. Keep Awake only prevents sleep.
On a Mac, the warnings are notifications, and macOS asks once whether Dictate Safe may show them. If you declined, allow them in System Settings → Notifications.
While Keep Awake is running, the floating icon’s label and the menu bar menu show the time left. If neither does, it is not running.
A dark or locked screen is not sleep. With Keep the screen on and unlocked switched off, the screen goes dark and locks as usual while the computer stays awake behind it, and the task keeps running.