Is it better to turn off iMac every night or leave on indefinitely?

Shutting a Mac down routinely actually causes more trouble than it's worth. If you're not going to use it for several days, ok, but nightly is not a good idea.

A sleeping iMac uses less energy than a night light.

If that's not enough information:

If you let your Mac sleep 8 hours a day it would consume about 3 kWh a year at a cost of almost certainly less than a dollar. But what if you were to shut it down and save the $1 a year? Consider the fact starting a Mac from a shutdown condition uses more energy than it consumes in a steady state once it finishes starting up, so at the end of the year you might have consumed more energy and spent more money. Calculating the break-even point is difficult to impossible because it's contingent on the programs you load when logging in and a lot of other variables, but it's almost certain to be more than just letting it consume the energy it does while sleeping eight to twelve hours overnight. Then there's the question of thermal cycling a cold Mac up to operating temperature and back down again, routinely, which arguably affects any piece of electronic equipment, leading to parts replacement or accelerated age-related failures.

Apple thinks about these things. They don't want people to have to bother with such minutia.

Still not enough information? What do you suppose people do with their iPhones? Case closed.

Still not enough information? Read this Product Environmental Report.

Same drill here. If I am leaving the house, or know that I will not be using the Macs for more than an hour, I power them down — particularly overnight. Been doing this for a long time, including work mandated equipment shutdowns before leaving the office. No equipment, or internal drive failures have occurred over the years.

FWIW, I have been shutting down computers every night

since my first MacPlus and my work IBM PC AT and have

never caused any harm to any system.

FWIW, if it was designed properly and manufactured properly,

it should not be an issue. This is based on my experience

as an electronics design engineer and actually performing thermal

stress testing.

That hour window is Mac sleep time. Based on my own work habits, if I am not in front of a Mac within an hour, then I have left the building, or engaged in something that will keep me away from the computer beyond a practical time (for me) to leave them running.

What matters is what works for you.

When you're not actively using a Mac it's best to simply ignore it.

Leaving Energy Saver settings at their defaults will ensure it doesn't consume energy needlessly.

Just to add, I don't even bother to put my Macs to sleep. I just turn off the displays. The only time my Macs are shutdown is when we lose AC due to a power outage.

Welcome.