Whats reasonable for suggestion?

[Yqatuba]

Could someone using it say "you look really tired (which is probably true if they've been fighting) why don't you take a nap" and have it work?

[deuxhero]

Compare to Deep Slumber, which most classes get on the same level, which has making targets sleep as it's explicit purpose. Deep Slumber gets AoE while this has longer duration and shorter time. However Sleep/Deep Slumber appear to be harder to break than "normal" sleep, which is what suggestion would put them in, and doing it in the middle of a battle isn't "reasonable". I'd go with yes, but

1: it wouldn't be trivial to walk up and CDG them like with the spells designed to sleep: The DC is still 10 higher than awake, but that's still decently doable if your stealth isn't reasonable.
2: They'd have to be the only combatant on their side. Sleeping during a battle isn't reasonable.
3: Sleeping on the floor isn't reasonable for most humanoids, they'd need a bed (though you could guide them to one, or the possibility of one). Factors such as night time (for a non-nocturnal creature) or the fatigue condition would make it reasonable enough for the save penalty.

Practically speaking you should include something about taking off uncomfortable armor on top of it. You can easily fit than and sleep into a sentence. "You're pretty tired, you want to take off that heavy armor and go sleep for hours in that nice bed."

[Yqatuba]

I know this is an old thread but I thought of something: what if they say "that armor looks really uncomfortable, why don't you take it off" would that work? After all, I imagine plate mail is pretty damn uncomfortable.

[Klorox]

So long as asking somebody to delay a red dragon for a round or two is deemed to 'sound reasonable", and in some editions, it was, there's pretty little that can't be achieved with a little verbal sugar coating.

[blahpers]

Suggestion is one of those really big levers that the game text's ambiguity leaves for the GM to adjust.

I rather like this Reddit post suggesting (heh) that "reasonable" be translated as "something that the character might agree to do while totally blitzed, possibly waking up later and going 'oh Desna, what did I do last night?'". In other words, judgment is severely impaired but not completely destroyed.

"That armor looks really uncomfortable" = sure. Maybe even word it such that it's clunky and hampering the victim's movements.

"Hold off that red dragon for a round or two" = heck yes for a capable warrior who might actually survive a round or two, heck no for Shrimpo the 1st-Level Commoner.

In some cases, "reasonable" may be extremely flexible, such as the victim being particularly foolhardy already. (Think "Have a drink!" from Twister.)

[Yqatuba]

What about a Zon-Kothon worshipper suggesting "Come visit my soundproof basement.."