It seems to me that the reason the rules don't include a penalty is that such a penalty is neither fun nor balanced. It may be realistic (though I have seen claims by some that one can, in fact, sleep comfortably in armour), but that doesn't balance the equation.
If the party is never going to be ambushed whilst sleeping, then the idea that they take off their armour to sleep can be handwaved away.
If they are, on occasion, going to be ambushed whilst sleeping, after the first time it happens, you might as well automatically impose the penalty on paladins and most fighters. After the first time they get attacked without their heavy armour, assuming they survive, they're probably never going to sleep without it again. If they don't survive, expect the party's replacement defender to be a Warden, who gets to sleep in his light armour and retain all of his AC without penalty.
If they're going to be frequently ambushed whilst sleeping, then either the DM is rather vicious or the party should be much more careful about where they choose to sleep. Whichever the case, they're going to be dealing with the penalties all of the time, because the alternative is worse.
A paladin, for instance, likely goes into battle normally wearing plate and a heavy shield. He likely hasn't put much into dexterity or intelligence, as aside from shoring up what's going to be a hopelessly low Reflex regardless of what he does, the abilities do little for him. It's entirely possible that neither score is higher than 10.
Leaving enhancement bonuses, armour specialization feats, and masterwork armour aside for the moment, a paladin in sleepwear is probably losing 10 points of AC. His powers and class features are all designed to make it more tempting to monsters to attempt to hit him (despite his usually high-end AC). Combining the two makes combat ridiculously lethal for him. No monster will miss him. If he takes the time to grab his shield and spend a standard action (costing him a round of attacking, or an action point) to put it in place, he can gain another 2 AC, but that's still not much help.
If he's an unusually intelligent or dexterous paladin, he may have a bonus that offsets the loss of armour to some small extent. 1 point of AC, maybe 2.
The rest of the party suffers as well. The paladin is more likely to fall in combat than usual, leaving the rest of the party facing more damage and less protection. More healing resources will have to got the paladin's way than usual, leaving less for everyone else.
It only gets worse for heavy armour users the higher you go, once you start factoring in enhancement bonuses, specialization feats, and their reliance on masterwork armour. A level 30 paladin is never going to take off his +6 Godplate. Give up 20+ points of AC? Forget it. Monsters 20 levels below him would have trouble missing.
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I think the best way to handle it, if you want to handle it at all, is to come at it from the opposite direction.
Assume that the rules already take into account the cost of sleeping in armour. Instead of penalizing them for wearing it, reward them if they opt not to.
Grant them extra healing surges (or some other bonus), for having had a more comfortable rest. Then they have the option of continuing to wear their armour, and having the AC and healing surges the rules assume they do, or of risking danger in the night in favour for advantage the next day.
