Yes - sleep mode lengthens a drive's life
It sounds like you might not know about this site, from one of your countrymen, which is the single best source for answers to questions like this on the entire Net - and I mean the very best.
Here, he deals with that very question:
http://www.dansdata.com/gz060.htm
here's the relevant passage from that article:
No spin down is a slow, but sure, kiss of death for consumer drives. They can last a long time if they spend most of their life in sleep mode - which they will, in many network storage situations. If their controller lets them spin down.
Keep a cheap drive spinning all day, and you not only waste watts and add more noise to your life, but should also expect the drive to fail in a couple of years, if not sooner. It may still last for ages, but that's not the way to bet.
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Not only true of cheap drives.
This guy should be known down there as a national treasure. Better than anything we in the US have to offer.
Enjoy.