This one was inspired by a forum post.
Basically, what's the thing with sleeping in armor? Easy, hard, impossible? No penalties, big penalties?
Sleeping in Armor in the RAW
Basic Set doesn't directly address sleeping in armor, or sleep quality.
DF just chucks the idea of a penalty out the window. You can get used to sleeping in armor, adventurers will expect to sleep in armor and thus can just go ahead and do that. It's not unreasonable; if you're in a dangerous area, expecting attack at any time, and will have almost no notice to get ready for violence, you will take less luxury to have more security.
Instant Armor has some rules for sleeping in armor that are a bit more harsh, giving some penalties that are outlined in this thread.
Both of the latter two make the assumption that the rules for sleep and FP assume restful sleep under good conditions. What if we change the baseline to "moderately restful sleep under dangerous conditions?" In other words, let's assume the rules assume sleeping outdoors, in armor, under adventuring conditions. So sleeping without armor, or sleeping in good conditions, is better than the baseline.
Then, we can give a bonus to good sleep.
In other words, if you want PCs to sometimes not sleep in armor, bribe them with a reward for taking that risk.
Here are three options I brainstormed.
More Efficient Sleep
This approach makes sleep without armor more efficient.
Sleeping without armor: Stripped down to soft, weather-appropriate skivvies, etc. with a bedroll and all of that. Sleep counts as 10% longer, round up. (7 hours counts as 8, 5 as 6, etc.)
Sleeping without armor, in a bed: Sleeping in a good bed, with pajamas, etc. the whole thing. Cheap common inn rooms don't count, but private rooms do. +20% sleep value.
Wake Up Refreshed
This approach makes you more awake and alert for a while if you sleep without armor.
Sleeping without armor: +1 to any IQ or Per rolls for 1 hour after waking.
Sleeping without armor, in a bed: +1 to any IQ or Per rolls for 2 hours after waking.
These assume a full amount of sleep (usually 8 hours.)
Wake Up With More Energy
This approach gives you more FP in return for sleeping without armor. Mages will definitely appreciate this, and it encourages less-armored martial artists who depend on FP-based chi powers.
Sleeping without armor: +1 FP over and above your maximum.
Sleeping without armor, in a bed: +2 FP over and above your maximum.
Additional FP last until spent, or until you go to sleep. It is not cumulative from day to day, and assumes a full night's worth of sleep (8 hours unless modified by advantages or disadvantages. So the most you can have is +2 FP, for bed rest.)
I haven't tried any of these - they are mainly just examples of rewarding the behavior you want to see instead of punishing the behavior everyone wants as a default. It encourages people to take the risk of fighting without armor in order to get a reward for doing so.
Skyrim does something like this. You literally never need to sleep. You can stay awake the whole game. But, if you do sleep, for a while afterward your skills improve at a higher rate. If you sleep in a bed you own, especially with your spouse (if you get married), you get an even bigger bonus. One of the downsides to a curse you can pick up in the game is that you can't benefit from sleep.
I've used this approach before with some moderate success. I give bonuses to PCs who spend extra on Carousing rolls or on upkeep in town in my DF game. I give people bonuses for cleaning up and not dressing in full armor when they visit NPCs for a social call. I reward extra bonuses to NPC hirelings with extra Loyalty and more positive feelingsin general. Some of them are also linked to punishments if you undercut the base, just like insufficient sleep is still an issue in GURPS DF even if sleeping in armor is just assumed as a base.
It's just a general approach of rewarding the behavior you want to encourage, instead of inflicting a baseline penalty on something everyone will do anyway.
As always, if you've got a better way to do this, or have tried something with good results (or even bad results), let me know in the comments!