Q:

How does Leia Organa know carbon freezing causes hibernation sickness?

In the Star Wars universe, there exists a carbon freezing process that has been used before to transport frozen gas over long distances.  In Cloud City, the carbon freezing process is used for the first time to encase a living person in suspended hibernation.
When Leia rescues Han Solo from his carbonite prison, Leia comments on the onset of hibernation sickness that Han is experiencing:

Leia: "Just relax for a moment. You're free of the carbonite. Shh. You have hibernation sickness."
Han: "I can't see."
Leia: "Your eyesight will return in time."

However, earlier, Lando Calrissian comments on how carbon freezing has never (to his knowledge) been used on humans before:

"Lord Vader, we only use this facility for carbon freezing. If you put him in there it might kill him."

How does Leia know that Han is experiencing hibernation sickness from being frozen in carbonite, if Han is the first person in history to go through that process?  Even if hibernation sickness is not exclusive to carbon freezing, how does Leia know that it is specifically what Han is experiencing?

A:

I think I found a pretty good quote to answer my own question.
Immediately after freezing Han Solo in carbonite, Darth Vader remarks to Lando:

Vader: "Well, Calrissian? Did he survive?"
Lando: "Yes, he's alive...and in perfect hibernation."

If we can assume that Leia is within earshot of this conversation (which she most likely would be, as she is still at this point in the same room as far as I can remember), we now know that Leia knows that Han Solo is now in a form of hibernation.  Since Leia already knows about the effects of hibernation sickness, it makes perfect sense that she would "put two and two together" by the time that she un-freezes Han and figure out that Han would, to the best of her knowledge, be experiencing a form of hibernation sickness.
As an interesting side note, Leia might already possess a limited knowledge of some effects of carbon freezing (in the Legends Canon, anyway) as the Alderaanian Medical Association has done at least limited research on it.

A:

This is never sufficiently explained in Disney Canon. Carbonite Freezing of people has been done before, but there's no evidence that Leia would have known about the results or effects.
However, in Legends Canon, this can be handwaved pretty easily.
Leia explains that Han is suffering from Hibernation Sickness, but does not specify that it is specific to, or only caused by being frozen in carbonite.
In fact, Hibernation Sickness is caused by being in suspended animation. Wookieepedia mentions a few methods of being put into suspended animation, such as sleeper ships and Sith techniques.
In Legends canon, since this is not a new concept or affliction, it's entirely possible/likely that she simply knew what sort of effects being in suspended animation would cause.

A:

We don't know for sure, but it's apparent that some time has passed between the end of Empire Strikes Back, when Han was frozen, and the beginning of RotJ, when he is rescued. Boba Fett delivers Han to Jabba, Jabba has time to make him into a decoration in his palace. Leia and company also had to plan the rescue operation, which obviously began at least some time before RotJ starts, because Lando is already working as a guard in Jabba's palace at the start of the movie. I think it's safe to assume that just like every other aspect of the rescue was planned, they also had to have some idea of what shape Han would be in after being unfrozen and that Leia did some studying up on carbonite freezing during that time to answer that question.

