Screen goes black when calling System Events?

tell application "Firefox"
	set the clipboard to "i"
	tell application "System Events"
		sleep 3
	end tell
end tell

This script turns off the screen, locks it, enters energy saver mode or something. Yes I know, it shouldn’t, but it does.

I have tried killing System Events from Activity Monitor, but it didn’t change anything.

Same result both when executed from Script Editor as well as command line/osascript.

Can I somehow restart the AS runtime environment?

The System Events sleep command puts the computer to sleep. What do you expecct it to do?

That’s exactly what the sleep command does.
Or am I missing something?

Sorry, I was writing some Bash just before and used sleep there, of course it is delay in AS…

For delay you don’t send it to system events as it is a built in AppleScript command. Remove the nested tell statements